Ink-jet printing method

ABSTRACT

The present invention relates to an ink jet printing method for printing images or characters on a recording medium using a water-based ink, in which the water-based ink includes pigment-containing water-insoluble polymer particles A, water-insoluble polymer particles B, an organic solvent C and water; the water-insoluble polymer particles A includes a water-insoluble polymer (a) containing constitutional units derived from an ionic monomer (a-1), an aromatic ring-containing hydrophobic monomer (a-2) and a specific hydrophilic monomer (a-3), the constitutional unit derived from the hydrophilic monomer (a-3) being present in an amount of from 13 to 45% by mass; the organic solvent C includes one or more organic solvents having a boiling point of 90° C. or higher, and has an average boiling point of 250° C. or lower as a weighted mean value; and the recording medium has a water absorption of from 0 to 10 g/m2 as measured in a pure water contact time of 100 ms. The ink-jet printing method is excellent in spreading of the dot size, optical density, gloss and rub fastness when printed on a low-water absorbing recording medium.

FIELD OF THE INVENTION

The present invention relates to an ink-jet printing method and awater-based ink for ink-jet printing.

BACKGROUND OF THE INVENTION

In ink-jet printing methods, droplets of ink are directly projected ontoa recording medium from very fine nozzles and allowed to adhere to therecording medium, to form images or characters. The ink-jet printingmethods have become rapidly spread because of various advantages such aseasiness of full coloration, low cost, capability of using a plain paperas the recording medium, non-contact with printed images or characters,etc.

In recent years, in order to impart a good weathering resistance and agood water resistance to printed matters, an ink containing a pigment asa colorant has been extensively used.

On the other hand, there is an increasing demand for commercial printedmatters printed on recording media using a low-liquid absorbing coatedpaper such as an offset-coated paper, or a non-liquid absorbing resinfilm such as a polyvinyl chloride resin film, a polypropylene resin filmand a polyester resin film.

It is known that when images or characters are printed on the low-liquidabsorbing or non-liquid absorbing recording media by the ink-jetprinting methods, there tends to occur problems such as slow absorptionof liquid components, prolonged drying time owing to poor absorption,and deterioration in rub fastness early after printing. Also, it isknown that unlike a plain paper in which a pigment is likely to bepenetrated, the low-liquid absorbing or non-liquid absorbing recordingmedia tend to suffer from deposition of pigment particles remainingthereon which are directly susceptible to an external force, so that theimages or characters printed on these recording media tend to bedeteriorated in rub fastness even after being dried.

In order to solve these conventional problems, ink-jet printing methodsusing recording media having an ink-absorbing layer have been proposed.

For example, JP 2008-260279A discloses an ink-jet printing method usinga recording medium having a pigment-containing coating layer in which anamount of pure water transferred to a surface of the recording medium onwhich the coating layer is provided, and a pH value of the same surfaceof the recording medium are controlled to respective specific ranges,and images or characters are printed on such a surface of the recordingmedium using an ink containing a granular coloring material, an emulsionresin and a surfactant and having a pH value of 8 or more. In addition,JP 2008-260279A proposes a recording apparatus equipped with a dryingdevice.

However, the improvement in recording medium and recording apparatustends to have problems concerning costs, consumed powder, etc., andtherefore it is desirable to improve the composition of inks.

JP 2005-36202A discloses an ink-jet printing method using a water-basedink containing colorant-containing water-insoluble vinyl polymerparticles and a void-type glossy medium in which the water-insolublevinyl polymer is produced from a polyoxyethylene chain-containing(meth)acrylate monomer, a salt-forming group-containing monomer and ahydrophobic monomer.

JP 2008-101192A discloses an ink-jet printing method in which an inkconstituted of a solid component containing a colorant and a resin and aliquid component having a boiling point higher than that of water isejected to print images on a recording medium, and a ratio between thesolid component and the liquid component as well as the viscosity of theink are respectively controlled to specific ranges.

JP 2003-105234A discloses a water-based ink for ink-jet printing whichincludes a salt-forming group-containing polymer A, a salt-forminggroup-containing polymer B and/or a salt-forming group-free polymer C inwhich the amount of a salt-forming group per unit weight of the polymerB is smaller than that per unit weight of the polymer A.

SUMMARY OF THE INVENTION

The present invention relates to the following aspects [1] and [2].

[1] An ink-jet printing method for printing images or characters on alow-water absorbing recording medium using a water-based ink for ink-jetprinting, in which

the water-based ink includes pigment-containing water-insoluble polymerparticles A, water-insoluble polymer particles B, an organic solvent Cand water;

a water-insoluble polymer (a) constituting the water-insoluble polymerparticles A contains constitutional units derived from specificmonomers; and

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C.

[2] A water-based ink for ink-jet printing, including pigment-containingwater-insoluble polymer particles A, water-insoluble polymer particlesB, an organic solvent C and water, in which

a water-insoluble polymer (a) constituting the water-insoluble polymerparticles A contains constitutional units derived from specificmonomers; and

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C.

DETAILED DESCRIPTION OF THE INVENTION

The conventional technologies concerning the ink-jet printing methods asdescribed in JP 2008-260279A, JP 2005-36202A, JP 2008-101192A and JP2003-105234A have failed to satisfy the properties of good opticaldensity, and good gloss and rub fastness of the resulting printed imagesor characters.

The present invention relates to an ink-jet printing method that isexcellent in spreading of the dot size, good optical density, and goodgloss and rub fastness when printed on a low-water absorbing recordingmedium.

Meanwhile, the term “low-water absorption” as used in the presentinvention is intended to mean both concepts of low-water absorption andnon-water absorption.

The present inventors have noticed a water-based ink containingpigment-containing water-insoluble polymer particles, anotherwater-insoluble polymer particles and an organic solvent, and have foundthat when controlling the content of a constitutional unit derived froma hydrophilic monomer having a specific structure which constitutes thewater-insoluble polymer to a specific range, combining an organicsolvent having a specific boiling point therewith to prepare an ink, andprinting images or characters on a low-water absorbing recording mediumusing the ink, it is possible to solve and overcome the aboveconventional problems.

That is, the present invention relates to the following aspects [1] and[2].

[1] An ink-jet printing method for printing images or characters on arecording medium using a water-based ink for ink-jet printing, in which

the water-based ink includes pigment-containing water-insoluble polymerparticles A, water-insoluble polymer particles B, an organic solvent Cand water;

the water-insoluble polymer particles A includes a water-insolublepolymer (a) containing a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the formula (1);

wherein R¹ is a hydrogen atom or a methyl group; R² is a hydrogen atom,an alkyl group having not less than 1 and not more than 20 carbon atomsor a phenyl group whose hydrogen atom may be substituted with an alkylgroup having not less than 1 and not more than 9 carbon atoms; and mrepresents an average molar number of addition of ethyleneoxy groups andis a number of not less than 7 and not more than 100,

the constitutional unit derived from the hydrophilic monomer (a-3) beingpresent in an amount of not less than 13% by mass and not more than 45%by mass on the basis of whole constitutional units in thewater-insoluble polymer (a);

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C; and the recording medium has a water absorptionof not less than 0 g/m² and not more than 10 g/m² as measured in a purewater contact time of 100 ms.

[2] A water-based ink for ink-jet printing, including pigment-containingwater-insoluble polymer particles A, water-insoluble polymer particlesB, an organic solvent C and water, in which

the water-insoluble polymer particles A includes a water-insolublepolymer (a) containing a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the aboveformula (1), the constitutional unit derived from the hydrophilicmonomer (a-3) being present in an amount of not less than 13% by massand not more than 45% by mass on the basis of whole constitutional unitsin the water-insoluble polymer (a); and

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C.

According to the present invention, there is provided an ink-jetprinting method that is excellent in spreading of the dot size, opticaldensity, gloss and rub fastness when printed on a low-water absorbingrecording medium.

[Ink-Jet Printing Method]

The present invention relates to an ink-jet printing method for printingimages or characters on a recording medium using a water-based ink forink-jet printing, in which

the water-based ink includes pigment-containing water-insoluble polymerparticles A, water-insoluble polymer particles B, an organic solvent Cand water;

the water-insoluble polymer particles A include a water-insolublepolymer (a) containing a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the aboveformula (1), the constitutional unit derived from the hydrophilicmonomer (a-3) being present in an amount of not less than 13% by massand not more than 45% by mass on the basis of whole constitutional unitsin the water-insoluble polymer (a);

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C; and

the recording medium has a water absorption of not less than 0 g/m² andnot more than 10 g/m² as measured in a pure water contact time of 100ms.

According to the ink-jet printing method of the present invention, therecan be attained such an effect that when printed on a low-waterabsorbing recording medium by the method, the resulting printed imagesor characters are excellent in spreading of the dot size, opticaldensity, gloss and rub fastness. The reason therefore is considered asfollows although not clearly determined.

The pigment-containing water-insoluble polymer particles A contain aspecific amount of the constitutional unit derived from the hydrophilicpolymer represented by the above formula (1), and therefore is free fromaggregation in water and the organic solvent C having a boiling point of90° C. or higher and exhibits a reduced viscosity when dried. As aresult, it is considered that dots of the ink on a low-water absorbingrecording medium are spread, and the resulting printed images orcharacters are enhanced in optical density and gloss, and further theink can be prevented from suffering from increase in viscosity owing toinclusion of the water-insoluble polymer and therefore is excellent inejection property.

In addition, the organic solvent C used in the ink has a average boilingpoint boiling point of 250° C. or lower as a weighed mean value, so thatthe drying velocity of the organic solvent itself on the low-waterabsorbing recording medium becomes high, and the ink is free fromaggregation of the water-insoluble polymer particles. As a result, it isconsidered that the organic solvent is inhibited from being incorporatedbetween the aggregated particles, and therefore the resulting ink canexhibit a higher drying velocity and has enhanced in rub fastness.

<Water-Based Ink for Ink-Jet Printing>

The water-based ink for ink-jet printing according to the presentinvention includes the pigment-containing water-insoluble polymerparticles A, the water-insoluble polymer particles B, the organicsolvent C and water.

Meanwhile, the term “water-based” as used herein means that water hasthe largest content among components of a medium contained in the ink,and an aqueous medium used therefore may be constituted of not onlywater solely but also a mixed solvent containing water and one or morekinds of organic solvents.

[Pigment-Containing Water-Insoluble Polymer Particles A] (Pigment)

The pigment used in the present invention may be either an inorganicpigment or an organic pigment.

Examples of the inorganic pigment include carbon blacks and metaloxides. In particular, carbon blacks are preferably used for blackwater-based inks. The carbon blacks may include furnace blacks, thermallamp blacks, acetylene blacks and channel blacks.

Examples of the organic pigment include azo pigments, diazo pigments,phthalocyanine pigments, quinacridone pigments, isoindolinone pigments,dioxazine pigments, perylene pigments, perinone pigments, thioindigopigments, anthraquinone pigments, and quinophthalone pigments.

The hue of the organic pigment used in the present invention is notparticularly limited, and there may be used any chromatic color pigmenthaving a yellow color, a magenta color, a cyan color, a red color, ablue color, an orange color, a green color, etc.

(Water-Insoluble Polymer (a))

The water-insoluble polymer (a) constituting the pigment-containingwater-insoluble polymer particles A are used for dispersing the pigmentin an aqueous medium and maintaining the resulting dispersion in astable state, and contains a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the formula (1).

The water-insoluble polymer (a) may be produced by subjecting the ionicmonomer (a-1), the aromatic ring-containing hydrophobic monomer (a-2)and the hydrophilic monomer (a-3) represented by the formula (1) toaddition polymerization by conventionally known methods.

The ionic monomer (a-1) is used as a monomer component of thewater-insoluble polymer (a) upon producing the below-mentioned “waterdispersion of the pigment-containing water-insoluble polymer particlesA” (hereinafter also referred to as a “pigment water dispersion”) fromthe viewpoint of enhancing the dispersion stability of the pigment waterdispersion and storage stability of the water-based ink.

Examples of the ionic monomer (a-1) include anionic monomers andcationic monomers. Among these monomers, from the viewpoint of enhancingthe dispersion stability of the pigment water dispersion and storagestability of the water-based ink as well as from the viewpoint ofimproving the ejecting property of the ink, preferred are anionicmonomers.

Examples of the anionic monomers include carboxylic acid monomers,sulfonic acid monomers and phosphoric acid monomers.

Specific examples of the carboxylic acid monomers include acrylic acid,methacrylic acid, crotonic acid, itaconic acid, maleic acid, fumaricacid, citraconic acid and 2-methacryloyloxymethylsuccinic acid.

Specific examples of the sulfonic acid monomers include styrenesulfonicacid, 2-acrylamido-2-methylpropanesulfonic acid and3-sulfopropyl(meth)acrylate.

Specific examples of the phosphoric acid monomers includevinylphosphonic acid, vinyl phosphate, bis(methacryloxyethyl)phosphate,diphenyl-2-acryloyloxyethyl phosphate anddiphenyl-2-methacryloyloxyethyl phosphate.

Among the above anionic monomers, from the viewpoint of enhancing adispersion stability of the pigment water dispersion and a storagestability of the water-based ink, preferred are the carboxylic acidmonomers, more preferred are acrylic acid and methacrylic acid, and evenmore preferred is methacrylic acid.

The aromatic ring-containing hydrophobic monomer (a-2) is used as amonomer component of the water-insoluble polymer (a) from the viewpointof enhancing the dispersion stability of the pigment water dispersionand storage stability of the water-based ink as well as from theviewpoint of improving the rub fastness of the ink when printed on alow-water absorbing recording medium.

The aromatic ring-containing hydrophobic monomer (a-2) is at least onemonomer selected from the group consisting of a styrene-based monomer,an aromatic group-containing (meth)acrylate and a styrene-basedmacromonomer. Meanwhile, the term “(meth)acrylate” means both anacrylate and a methacrylate, and is hereinafter defined in the same way.

As the styrene-based monomer, from the viewpoint of enhancing adispersion stability of the pigment water dispersion and a storagestability of the water-based ink, preferred are styrene and 2-methylstyrene, and more preferred is styrene.

As the aromatic group-containing (meth)acrylate, from the viewpoint ofenhancing the dispersion stability of the pigment water dispersion andstorage stability of the water-based ink as well as from the viewpointof improving rub fastness of the ink when printed on a low-waterabsorbing recording medium, preferred are benzyl(meth)acrylate andphenoxyethyl(meth)acrylate, and more preferred is benzyl(meth)acrylate.

The styrene-based macromonomer is a compound having a number-averagemolecular weight of not less than 500 and not more than 100,000 whichcontains a polymerizable functional group at one terminal end thereof.The number-average molecular weight of the styrene-based macromonomer ispreferably not less than 1,000 and not more than 10,000 from theviewpoint of enhancing the dispersion stability of the pigment waterdispersion and storage stability of the water-based ink as well as fromthe viewpoint of improving the rub fastness of the ink when printed on alow-water absorbing recording medium. Meanwhile, the number-averagemolecular weight may be measured by gel permeation chromatography usingchloroform containing 1 mmol/L of dodecyldimethylamine as a solvent andpolystyrene as a reference standard substance.

Examples of the commercially available styrene-based macromonomerincludes AS-6(S), AN-6(S) and HS-6(S) (tradenames available fromToagosei Co., Ltd.), etc.

As the aromatic ring-containing hydrophobic monomer (a-2), from theviewpoint of enhancing dispersion stability of the pigment waterdispersion and storage stability of the water-based ink as well as fromthe viewpoint of improving rub fastness of the ink when printed on alow-water absorbing recording medium, preferred is at least one compoundselected from the group consisting of an aromatic group-containing(meth)acrylate and a styrene-based macromonomer, more preferred is thecombination of an aromatic group-containing (meth)acrylate and astyrene-based macromonomer, and even more preferred is the combinationof benzyl(meth)acrylate and a styrene-based macromonomer.

The hydrophilic monomer represented by the formula (1) is used as amonomer component of the water-insoluble polymer (a) from the viewpointof reducing viscosity of the ink upon evaporating the solvent therefrom,increasing the dot size of the ink and enhancing optical density andgloss of the ink when printed on a low-water absorbing recording mediumas well as from the viewpoint of improving the storage stability andejection property of the water-based ink:

wherein R¹ is a hydrogen atom or a methyl group; R² is a hydrogen atom,an alkyl group having not less than 1 and not more than 20 carbon atomsor a phenyl group whose hydrogen atom may be substituted with an alkylgroup having not less than 1 and not more than 9 carbon atoms; and mrepresents an average molar number of addition of ethyleneoxy groups andis a number of not less than 7 and not more than 100.

In the above formula (1), R¹ is a hydrogen atom or a methyl group, andpreferably a methyl group from the viewpoint of enhancing the dispersionstability of the pigment water dispersion and storage stability of thewater-based ink.

R² is a hydrogen atom, an alkyl group having not less than 1 and notmore than 20 carbon atoms or a phenyl group whose hydrogen atom may besubstituted with an alkyl group having not less than 1 and not more than9 carbon atoms. R² is preferably a hydrogen atom or an alkyl grouphaving not less than 1 and not more than 20 carbon atoms, morepreferably an alkyl group having not less than 1 and not more than 3carbon atoms and even more preferably a methyl group from the viewpointof enhancing the dispersion stability of the pigment water dispersionand storage stability of the water-based ink.

In the above formula (1), m is a number of not less than 7 and not morethan 100. From the viewpoint of reducing viscosity of the ink uponevaporating the solvent therefrom, increasing the dot size of the inkand enhancing the optical density and gloss of the ink when printed on alow-water absorbing recording medium as well as from the viewpoint ofimproving storage stability and ejection property of the water-basedink, m is preferably not less than 9, more preferably not less than 12,even more preferably not less than 16 and further even more preferablynot less than 20, and is also preferably not more than 90, morepreferably not more than 70, even more preferably not more than 55 andfurther even more preferably not more than 25.

From the viewpoint of enhancing optical density of the printed images orcharacters, m is preferably from 12 to 90, more preferably from 16 to 70and even more preferably from 20 to 55. Also, from the viewpoint ofenhancing gloss of the printed images or characters, m is preferablyfrom 12 to 90, more preferably from 25 to 90 and even more preferablyfrom 25 to 70. Thus, m is preferably not less than 12, more preferablynot less than 16, even more preferably not less than 20 and further evenmore preferably not less than 25, and is also preferably not more than90, more preferably not more than 70, even more preferably not more than55 and further even more preferably not more than 25.

From the total viewpoints, m is preferably from 12 to 90, morepreferably from 16 to 70, even more preferably from 20 to 55, andfurther even more preferably from 20 to 25.

Preferred examples of the monomer (a-3) represented by the above formula(1) include at least one monomer selected from the group consisting ofpolyethylene glycol mono(meth)acrylate, methoxy polyethylene glycolmono(meth)acrylate, octoxy polyethylene glycol mono(meth)acrylate andstearoyloxy polyethylene glycol mono(meth)acrylate. Among thesemonomers, more preferred is methoxy polyethylene glycolmono(meth)acrylate.

Specific examples of the commercially available monomer represented bythe above formula (1) include “NK Ester M-90G”, “NK Ester M-230G”, “NKEster M-450G” and “NK Ester M-900G” all available from Shin-NakamuraKagaku Kogyo Co., Ltd., and “LIGHT ESTER 041MA” available from KyoeishaChemical Co., Ltd.

The water-insoluble polymer (a) used in the present invention may alsocontain other monomers than the ionic monomer (a-1), the aromaticring-containing hydrophobic monomer (a-2) and the hydrophilic monomer(a-3) represented by the formula (1) unless the desired effects of thepresent invention are adversely affected.

Examples of the other monomers include alkyl(meth)acrylates having 1 to22 carbon atoms such as methyl(meth)acrylate and2-ethylhexyl(meth)acrylate, hydroxyalkyl(meth)acrylates such as2-hydroxyethyl(meth)acrylate and 3-hydroxyethyl(meth)acrylate, andsilicone-based macromonomers such as organopolysiloxanes having apolymerizable functional group at one terminal end thereof.

Upon production of the water-insoluble polymer (a), the contents of theabove monomers (a-1), (a-2) and (a-3) in a mixture containing thesemonomers (hereinafter also referred to merely as a “monomer mixture”)(contents of non-neutralized components; hereinafter defined in the sameway) or the contents of the constitutional units derived from themonomers (a-1), (a-2) and (a-3) in the water-insoluble polymer (a) areas follows.

The content of the ionic monomer (a-1) is preferably not less than 3% bymass, more preferably not less than 5% by mass and even more preferablynot less than 8% by mass, and is also preferably not more than 40% bymass, more preferably not more than 25% by mass and even more preferably20% by mass from the viewpoint of enhancing a dispersion stability ofthe pigment water dispersion and a storage stability of the water-basedink.

The content of the aromatic ring-containing hydrophobic monomer (a-2) ispreferably not less than 20% by mass, more preferably not less than 40%by mass and even more preferably not less than 50% by mass, and is alsopreferably not more than 80% by mass, more preferably not more than 75%by mass and even more preferably 70% by mass from the viewpoint ofenhancing the dispersion stability of the pigment water dispersion andstorage stability of the water-based ink as well as from the viewpointof improving rub fastness of the ink when printed on a low-waterabsorbing recording medium.

The content of the styrene-based macromonomer used as the monomer (a-2)is preferably not less than 5% by mass and more preferably not less than8% by mass, and is also preferably not more than 15% by mass and morepreferably not more than 12% by mass from the viewpoint of enhancing thedispersion stability of the pigment water dispersion and storagestability of the water-based ink as well as from the viewpoint ofimproving rub fastness of the ink when printed on a low-water absorbingrecording medium.

The content of the hydrophilic monomer (a-3) is preferably from 13 to45% by mass, more preferably from 15 to 35% by mass and even morepreferably from 18 to 25% by mass from the viewpoint of reducingviscosity of the ink upon evaporating the solvent therefrom, increasingthe dot size of the ink and enhancing optical density and gloss of theink when printed on a low-water absorbing recording medium as well asfrom the viewpoint of improving storage stability and ejection propertyof the water-based ink. Thus, the content of the hydrophilic monomer(a-3) is preferably not less than 15% by mass and more preferably notless than 18% by mass, and is also preferably not more than 35% by massand more preferably not more than 25% by mass.

The mass ratio of the component (a-1) to a sum of the component (a-2)and the component (a-3) {component (a-1)/[component (a-2)+component(a-3)]} is preferably not less than 0.03, more preferably not less than0.05 and even more preferably not less than 0.10, and is also preferablynot more than 0.50, more preferably not more than 0.30 and even morepreferably not more than 0.20 from the viewpoint of enhancing thedispersion stability of the pigment water dispersion and storagestability of the water-based ink.

(Production of Water-Insoluble Polymer (a))

The water-insoluble polymer (a) may be produced by copolymerizing theabove monomer mixture by known polymerization methods. Among thepolymerization methods, preferred is a solution polymerization method.

The organic solvent (a) used in the solution polymerization method isnot particularly limited, and is preferably at least one compoundselected from the group consisting of ketones having not less than 4 andnot more than 8 carbon atoms, alcohols, ethers and esters from theviewpoint of enhancing productivity of the below-mentioned waterdispersion of the pigment-containing water-insoluble polymer particlesA. Of these compounds, preferred are ketones having not less than 4 andnot more than 8 carbon atoms, and more preferred is methyl ethyl ketone.

The polymerization may be carried out in the presence of apolymerization initiator or a chain transfer agent. As thepolymerization initiator, preferred are azo compounds, and morepreferred is 2,2′-azobis(2,4-dimethylvaleronitrile).

As the chain transfer agent, preferred are mercaptans, and morepreferred is 2-mercaptoethanol.

The preferred polymerization conditions may vary depending upon the kindof polymerization initiator used, etc. The polymerization temperature ispreferably not lower than 50° C. and not higher than 90° C., and thepolymerization time is preferably not shorter than 1 h and not longerthan 20 h. Further, the polymerization is preferably conducted in aninert gas atmosphere such as nitrogen atmosphere and argon.

After completion of the polymerization reaction, the polymer thusproduced may be isolated from the reaction solution by a known methodsuch as reprecipitation and removal of solvent by distillation. The thusobtained polymer may be subjected to reprecipitation, membraneseparation, chromatography, extraction, etc., for removing unreactedmonomers, etc., therefrom.

The water-insoluble polymer (a) is preferably used as such in the formof a polymer solution without removing the organic solvent (a) used inthe polymerization reaction therefrom in order to use the organicsolvent (a) as the below-mentioned organic solvent (b) from theviewpoint of enhancing productivity of the water dispersion of thepigment-containing water-insoluble polymer particles A.

The solid content of a solution of the water-insoluble polymer (a) ispreferably not less than 30% by mass and more preferably not less than40% by mass, and is also preferably not more than 60% by mass and morepreferably not more than 50% by mass from the viewpoint of enhancingproductivity of the water dispersion of the pigment-containingwater-insoluble polymer particles A.

The weight-average molecular weight of the water-insoluble polymer (a)used in the present invention is preferably not less than 10,000, morepreferably not less than 15,000 and even more preferably not less than20,000 from the viewpoint of enhancing the dispersion stability of thepigment water dispersion and storage stability of the water-based ink,and is also preferably not more than 150,000, more preferably not morethan 100,000, even more preferably not more than 60,000 and further evenmore preferably not more than 40,000 from the viewpoint of reducingviscosity of the ink upon evaporating the organic solvent C therefrom,increasing the dot size of the ink and enhancing optical density andgloss of the ink when printed on a low-water absorbing recording mediumas well as from the viewpoint of improving storage stability andejection property of the water-based ink. Meanwhile, the weight-averagemolecular weight of the water-insoluble polymer may be measured by themethod described in Examples below.

(Production of Pigment-Containing Water-Insoluble Polymer Particles A)

The pigment-containing water-insoluble polymer particles A arepreferably used in the form of a pigment water dispersion, and used inthe water-based ink for ink-jet printing from the viewpoint of enhancingproductivity of the water-based ink.

The water dispersion of the pigment-containing water-insoluble polymerparticles A (pigment water dispersion) may be produced by the processincluding the following steps (1) and (2).

Step (1): subjecting a mixture of the water-insoluble polymer (a), theorganic solvent (b), the pigment and water (hereinafter also referred toas a “pigment mixture”) to dispersion treatment to obtain a dispersiontreatment product; and

Step (2): removing the organic solvent (b) from the dispersion treatmentproduct obtained in step (1) to obtain a pigment water dispersion.

<Step (1)>

In step (1), the mixture of the water-insoluble polymer (a), the organicsolvent (b), the pigment and water (pigment mixture) was subjected todispersion treatment to obtain the dispersion treatment product.

In step (1), first, the water-insoluble polymer (a), the organic solvent(b), the pigment and water, if required, together with a neutralizingagent, a surfactant and the like, are preferably mixed with each otherto obtain the pigment mixture. The order of addition of the respectivecomponents is not particularly limited, and it is preferred that thewater-insoluble polymer (a), the organic solvent (b), the neutralizingagent, water and the pigment are successively added in this order.

(Organic Solvent (b))

The organic solvent (b) is not particularly limited, and is preferablyselected from aliphatic alcohols having not less than 1 and not morethan 3 carbon atoms, ketones having not less than 4 and not more than 8carbon atoms, ethers, esters and the like. Of these organic solvents,from the viewpoint of improving wettability to the pigment, solubilityof the water-insoluble polymer (a) and adsorbing property of thewater-insoluble polymer (a) to the pigment, more preferred are ketoneshaving not less than 4 and not more than 8 carbon atoms, even morepreferred are methyl ethyl ketone and methyl isobutyl ketone, and evenmore preferred is methyl ethyl ketone.

The mass ratio of the water-insoluble polymer (a) to the organic solvent(b) [water-insoluble polymer (a)/organic solvent (b)] is preferably notless than 0.10, more preferably not less than 0.15 and even morepreferably not less than 0.20, and is also preferably not more than0.60, more preferably not more than 0.50 and even more preferably notmore than 0.40 from the viewpoint of improving wettability to thepigment and adsorbing property of the water-insoluble polymer (a) to thepigment.

(Neutralizing Agent)

In the present invention, from the viewpoint of enhancing the dispersionstability of the pigment water dispersion as well as the storagestability and ejection property of the water-based ink, there may beused a neutralizing agent. When using the neutralizing agent, the pHvalue of the pigment water dispersion is preferably controlled to notless than 7 and more preferably not less than 7.5, and is alsopreferably controlled to not more than 11 and more preferably not morethan 9.5.

Examples of the neutralizing agent include hydroxides of alkali metals,ammonia and organic amines.

Specific examples of the hydroxides of alkali metals include lithiumhydroxide, sodium hydroxide, potassium hydroxide and cesium hydroxide.Among these hydroxides of alkali metals, preferred is sodium hydroxide.

Specific examples of the organic amines include trimethylamine,ethylamine, diethylamine, triethylamine and triethanolamine.

As the neutralizing agent, from the viewpoint of enhancing thedispersion stability of the pigment water dispersion as well as thestorage stability and ejection property of the water-based ink,preferred are the hydroxides of alkali metals and ammonia, and morepreferred is the combination of sodium hydroxide and ammonia.

The neutralizing agent is preferably used in the form of an aqueousneutralizing agent solution from the viewpoint of acceleratingneutralization of the polymer in a sufficient and uniform manner. Fromthe same viewpoints, the concentration of the aqueous neutralizing agentsolution is preferably not less than 3% by mass, more preferably notless than 10% by mass and even even more preferably not less than 15% bymass, and is also preferably not more than 30% by mass and morepreferably not more than 25% by mass.

The mass ratio of the aqueous neutralizing agent solution to the organicsolvent (b) [aqueous neutralizing agent solution/organic solvent (b)] ispreferably not less than 0.01, more preferably not less than 0.05 andeven more preferably not less than 0.10, and is also preferably not morethan 0.50, more preferably not more than 0.30 and even more preferablynot more than 0.20 from the viewpoint of accelerating adsorption of thewater-insoluble polymer (a) to the pigment and neutralization of thepolymer, enhancing the dispersion stability of the pigment waterdispersion and storage stability of the water-based ink, reducingformation of coarse particles, and improving ejection property of thewater-based ink.

The neutralizing agent and the aqueous neutralizing agent solution maybe respectively used alone or in the form of a mixture of any two ormore kinds thereof.

The degree of neutralization of the water-insoluble polymer (a) ispreferably not less than 30 mol %, more preferably not less than 40 mol% and even more preferably not less than 50 mol %, and is alsopreferably not more than 300 mol %, more preferably not more than 200mol % and even more preferably not more than 150 mol % from theviewpoint of enhancing dispersion stability of the pigment waterdispersion and storage stability of the water-based ink, reducingformation of coarse particles, and improving ejection property of thewater-based ink.

The degree of neutralization of the polymer by the hydroxides of alkalimetals among the above neutralizing agents is preferably not less than30 mol %, more preferably not less than 40 mol % and even morepreferably not less than 50 mol %, and is also preferably not more than150 mol %, more preferably not more than 125 mol % and even morepreferably not more than 100 mol %.

The degree of neutralization of the polymer by the neutralizing agentmeans the value calculated by dividing a mole equivalent amount of theneutralizing agent by a molar amount of an anionic group in thewater-insoluble polymer (a).

(Contents of Respective Components, etc.)

The content of the pigment in the pigment mixture is preferably not lessthan 10% by mass, more preferably not less than 12% by mass and evenmore preferably not less than 15% by mass, and is also preferably notmore than 30% by mass, more preferably not more than 25% by mass andeven more preferably not more than 20% by mass from the viewpoint ofenhancing the dispersion stability of the pigment water dispersion andstorage stability and ejection property of the water-based ink as wellas from the viewpoint of improving productivity of the pigment waterdispersion.

The content of the water-insoluble polymer (a) in the pigment mixture ispreferably not less than 2.0% by mass, more preferably not less than4.0% by mass and even more preferably not less than 5.0% by mass, and isalso preferably not more than 15% by mass, more preferably not more than10% by mass and even more preferably not more than 8.0% by mass from theviewpoint of enhancing the dispersion stability of the pigment waterdispersion and storage stability and ejection property of thewater-based ink.

The content of the organic solvent (b) in the pigment mixture ispreferably not less than 10% by mass, more preferably not less than 12%by mass and even more preferably not less than 15% by mass, and is alsopreferably not more than 35% by mass, more preferably not more than 30%by mass and even more preferably not more than 25% by mass from theviewpoint of improving wettability to the pigment and adsorbing propertyof the water-insoluble polymer (a) to the pigment.

The content of water in the pigment mixture is preferably not less than40% by mass, more preferably not less than 45% by mass and even morepreferably not less than 50% by mass, and is also preferably not morethan 75% by mass, more preferably not more than 70% by mass and evenmore preferably not more than 65% by mass from the viewpoint ofenhancing dispersion stability of the pigment water dispersion as wellas from the viewpoint of improving productivity of the pigment waterdispersion.

The mass ratio of the pigment to the water-insoluble polymer (a)[pigment/water-insoluble polymer (a)] is preferably from 30/70 to 90/10,more preferably from 50/50 to 85/15 and even more preferably from 70/30to 80/20 from the viewpoint of reducing viscosity of the ink uponevaporating the solvent therefrom, increasing the dot size of the inkand enhancing optical density and gloss of the ink when printed on alow-water absorbing recording medium as well as from the viewpoint ofimproving the storage stability and ejection property of the water-basedink. Thus, the mass ratio of the pigment to the water-insoluble polymer(a) [pigment/water-insoluble polymer (a)] is preferably not less than30/70, more preferably not less than 50/50 and even more preferably notless than 70/30, and is also preferably not more than 90/10, morepreferably not more than 85/15 and even more preferably not more than80/20.

(Dispersion of Pigment Mixture)

In step (1), the pigment mixture is further dispersed to obtain adispersion treatment product. The dispersing method for obtaining thedispersion treatment product in step (1) is not particularly limited.The pigment particles may be finely atomized into fine particles havinga desired average particle size only by a substantial dispersiontreatment. Preferably, the pigment mixture is first subjected to apreliminary dispersion treatment, and then to the substantial dispersiontreatment by applying a shear stress thereto so as to control theaverage particle size of the obtained pigment particles to a desiredvalue.

The temperature used in the preliminary dispersion treatment in step (1)is preferably not lower than 0° C., and is also preferably not higherthan 40° C., more preferably not higher than 30° C. and even morepreferably not higher than 20° C. The dispersing time is preferably notshorter than 0.5 h and more preferably not shorter than 1 h, and is alsopreferably not longer than 30 h, more preferably not longer than 10 hand even more preferably not longer than 5 h.

When subjecting the pigment mixture to the preliminary dispersiontreatment, there may be used ordinary mixing or stirring devices such asanchor blades and disper blades. Of these devices, preferred arehigh-speed stirring mixers.

As a means for applying shear stress to the pigment mixture in thesubstantial dispersion treatment, there may be used, for example,kneading machines such as roll mills and kneaders, high-pressurehomogenizers such as “Micro Fluidizer” (available from MicrofluidicsInc.), and media-type dispersers such as paint shakers and beads mills.Examples of the commercially available media-type dispersers include“Ultra Apex Mill” (available from Kotobuki Industries Co., Ltd.) and“Pico Mill” (available from Asada Iron Works Co., Ltd.). These devicesmay be used in combination of any two or more thereof. Among thesedevices, the high-pressure homogenizers are preferably used from theviewpoint of reducing the particle size of the pigment.

When conducting the substantial dispersion treatment using thehigh-pressure homogenizers, it is possible to adjust the particle sizeof the pigment to a desired value by controlling the pressure used inthe substantial dispersion treatment or frequency of passing thedispersion through the devices.

The pressure used in the substantial dispersion treatment is not lessthan 60 MPa, more preferably not less than 100 MPa and even morepreferably not less than 130 MPa, and is also preferably not more than200 MPa, more preferably not more than 180 MPa and even more preferablynot more than 160 MPa from the viewpoint of enhancing productivity andeconomy.

The frequency of passing the dispersion through the devices ispreferably not less than 3 passes, more preferably not less than 10passes and even more preferably not less than 15 passes, and is alsopreferably not more than 30 passes and more preferably not more than 25passes.

<Step (2)>

In step (2), the organic solvent (b) is removed from the dispersiontreatment product obtained in step (1) to obtain a pigment waterdispersion. The removal of the organic pigment (b) may be conducted byconventionally known methods.

From the viewpoint of suppressing generation of aggregates in the stepof removing the organic solvent (b) and enhancing the dispersionstability of the pigment water dispersion as well as storage stabilityand ejection property of the water-based ink, it is preferred that wateris added to the dispersion treatment product obtained in step (1) beforeremoving the organic solvent (b) therefrom to control the mass ratio ofthe organic solvent (b) to water (organic solvent (b)/water).

The mass ratio of the organic solvent (b) to water (organic solvent(b)/water) is preferably not less than 0.08 and more preferably not lessthan 0.10, and is also preferably not more than 0.40 and more preferablynot more than 0.20.

The concentration of non-volatile components (solid content) in thepigment water dispersion after controlling the mass ratio of the organicsolvent (b) to water (organic solvent (b)/water) is preferably not lessthan 5% by mass, more preferably not less than 8% by mass and even morepreferably not less than 10% by mass, and is also preferably not morethan 30% by mass, more preferably not more than 20% by mass and evenmore preferably not more than 16% by mass, from the viewpoint ofsuppressing generation of aggregates in the step of removing the organicsolvent (b) as well as from the viewpoint of improving productivity ofthe pigment water dispersion. Meanwhile, a part of water contained inthe pigment water dispersion may be removed simultaneously with theorganic solvent (b).

Examples of the apparatus for removing the organic solvent (b) in thestep (2) include a thin film distillation apparatus such as a batchsimple distillation device, a reduced pressure distillation device and aflash evaporator, a rotary distillation device and a stirringevaporator. Among these apparatuses, from the viewpoint of efficientremoval of the organic solvent (b), preferred are a rotary distillationdevice and a stirring evaporator, more preferred is a rotarydistillation device, and even more preferred is a rotary evaporator.

The temperature of the dispersion treatment product upon removal of theorganic solvent (b) may appropriately vary depending upon the kind oforganic solvent (b) to be removed. The temperature of the dispersiontreatment product upon removal of the organic solvent (b) as measuredunder reduced pressure is preferably not lower than 20° C., morepreferably not lower than 25° C. and even more preferably not lower than30° C., and is also preferably not higher than 80° C., more preferablynot higher than 70° C. and even more preferably not higher than 65° C.

The pressure of the reaction system upon removal of the organic solvent(b) is preferably not less than 0.01 MPa, more preferably not less than0.02 MPa and even more preferably not less than 0.05 MPa, and is alsopreferably not more than 0.5 MPa, more preferably not more than 0.2 MPaand even more preferably not more than 0.1 MPa.

The time required for removal of the organic solvent (b) is preferablynot shorter than 1 h, more preferably not shorter than 2 h and even morepreferably not shorter than 5 h, and is also preferably not longer than24 h, more preferably not longer than 12 h and even more preferably notlonger than 10 h.

The removal of the organic solvent (b) is continued until the solidcontent of the resulting dispersion treatment product reaches preferablynot less than 10% by mass and more preferably not less than 20% by mass,and also until the solid content of the resulting dispersion treatmentproduct reaches preferably not more than 35% by mass and more preferablynot more than 30% by mass.

The resulting concentrated product is preferably subjected tocentrifugal separation treatment, and the thus separated liquid layerportion is filtered to obtain the pigment water dispersion. Morespecifically, the thus obtained concentrated product is preferablysubjected to centrifugal separation treatment to separate theconcentrated product into a liquid layer portion and a solid portion,thereby recovering the liquid layer portion. The thus recovered liquidlayer portion is mainly formed of a dispersion containing thepigment-containing water-insoluble polymer particles A dispersed inwater, whereas the solid portion is mainly formed of coarse particlesproduced owing to defective dispersion or aggregation. Therefore, thepigment water dispersion is obtained from the liquid layer portion.

Also, from the viewpoint of preventing the resulting pigment waterdispersion from being dried and suffering from putrefaction thereof, ahumectant such as glycerin, an antiseptic agent or a mildew-proof agentmay be added to the pigment water dispersion.

The organic solvent (b) is preferably substantially completely removedfrom the thus obtained pigment water dispersion. However, the residualorganic solvent (b) may be present in the pigment water dispersionunless the objects and effects of the present invention are adverselyaffected by the residual organic solvent. The content of the residualorganic solvent (b) in the resulting pigment water dispersion ispreferably not more than 0.1% by mass and more preferably not more than0.01% by mass.

The concentration of the non-volatile components (solid content) in theresulting pigment water dispersion is preferably not less than 10% bymass and more preferably not less than 15% by mass, and is alsopreferably not more than 30% by mass and more preferably not more than25% by mass from the viewpoint of enhancing dispersion stability of thepigment water dispersion as well as from the viewpoint of facilitatingpreparation of the water-based ink.

In the thus-obtained pigment water dispersion, solid components of thepigment and the water-insoluble polymer (a) are dispersed in water as amain dispersing medium.

The average particle size of the pigment-containing water-insolublepolymer particles A in the pigment water dispersion is preferably notless than 40 nm, more preferably not less than 60 nm and even morepreferably not less than 75 nm, and is also preferably not more than 150nm, more preferably not more than 120 nm, even more preferably not morethan 110 nm and further even more preferably not more than 100 nm fromthe viewpoint of accelerating adsorption of the water-insoluble polymer(a) to the pigment and neutralization of the polymer to improvedispersion stability of the pigment water dispersion and, as a result,from the viewpoint of suppressing formation of coarse particles andimproving ejection property of the water-based ink, and further from theviewpoint of reducing the viscosity of the ink upon evaporating thesolvent therefrom, increasing the dot size of the ink and enhancingoptical density and gloss of the ink when printed on a low-waterabsorbing recording medium.

Meanwhile, the average particle size of the pigment-containingwater-insoluble polymer particles A may be measured by the methoddescribed in Examples below.

The average particle size of the pigment-containing water-insolublepolymer particles A in the water-based ink is preferably not less than40 nm, more preferably not less than 60 nm and even more preferably notless than 75 nm, and is also preferably not more than 150 nm, morepreferably not more than 120 nm, even more preferably not more than 110nm and further even more preferably not more than 100 nm from theviewpoint of accelerating adsorption of the water-insoluble polymer (a)to the pigment and neutralization of the polymer to improve dispersionstability of the pigment water dispersion and, as a result, from theviewpoint of suppressing formation of coarse particles and improvingejection property of the water-based ink, and further from the viewpointof reducing the viscosity of the ink upon evaporating the solventtherefrom, increasing the dot size of the ink and enhancing opticaldensity and gloss of the ink when printed on a low-water absorbingrecording medium.

(Amount and Content of Components Compounded in Water-Based Ink forInk-Jet Printing)

The amount of the pigment water dispersion compounded in the water-basedink is preferably not less than 10% by mass, more preferably not lessthan 15% by mass and even more preferably not less than 20% by mass fromthe viewpoint of promoting drying of the ink on the surface of paper andimproving rub fastness and optical density of the ink when printed on alow-water absorbing recording medium, and is also preferably not morethan 50% by mass, more preferably not more than 40% by mass and evenmore preferably not more than 35% by mass from the viewpoint of reducingviscosity of the ink upon evaporating the solvent therefrom, increasingthe dot size of the ink and enhancing optical density and gloss of theink when printed on a low-water absorbing recording medium as well asfrom the viewpoint of improving storage stability and ejection propertyof the water-based ink.

The content of the pigment-containing water-insoluble polymer particlesA in the water-based ink is preferably not less than 1% by mass, morepreferably not less than 3% by mass and even more preferably not lessthan 4% by mass from the viewpoint of promoting drying of the ink on thesurface of paper and improving rub fastness and optical density of theink when printed on a low-water absorbing recording medium, and is alsopreferably not more than 20% by mass, more preferably not more than 10%by mass and even more preferably not more than 8% by mass from theviewpoint of reducing viscosity of the ink upon evaporating the solventtherefrom, increasing the dot size of the ink and enhancing opticaldensity and gloss of the ink when printed on a low-water absorbingrecording medium as well as from the viewpoint of improving storagestability and ejection property of the water-based ink.

The content of the pigment in the water-based ink is preferably not lessthan 1% by mass, more preferably not less than 2% by mass and even morepreferably not less than 3% by mass from the viewpoint of promotingdrying of the ink on the surface of paper and improving optical densityof the ink when printed on a low-water absorbing recording medium, andis also preferably not more than 15% by mass, more preferably not morethan 10% by mass and even more preferably not more than 6% by mass fromthe viewpoint of reducing viscosity of the ink upon evaporating thesolvent therefrom, increasing the dot size of the ink and enhancingoptical density and gloss of the ink when printed on a low-waterabsorbing recording medium as well as from the viewpoint of improvingstorage stability and ejection property of the water-based ink.

The content of the water-insoluble polymer (a) in the water-based ink ispreferably not less than 0.5% by mass, more preferably not less than0.8% by mass and even more preferably not less than 1% by mass from theviewpoint of enhancing storage stability and ejection property of thewater-based ink and improving rub fastness of printed images orcharacters when printed on a low-water absorbing recording medium, andis also preferably not more than 6% by mass, more preferably not morethan 3% by mass and even more preferably not more than 2% by mass fromthe viewpoint of reducing viscosity of the ink upon evaporating thesolvent therefrom, increasing the dot size of the ink and enhancingoptical density and gloss of the ink when printed on a low-waterabsorbing recording medium as well as from the viewpoint of improvingstorage stability and ejection property of the water-based ink.

[Water-Insoluble Polymer Particles B]

The water-based ink for ink-jet printing as used in the presentinvention contains the water-insoluble polymer particles B from theviewpoint of promoting drying of the ink on the surface of a recordingmedium (paper) for ink-jet printing and improving rub fastness ofprinted images or characters when printed on a low-water absorbingrecording medium.

As the water-insoluble polymer particles B, there may be used optionalwater-insoluble polymer particles. The water-insoluble polymer particlesB may be used in the form of a dispersion prepared by dispersing thewater-insoluble polymer particles B in water as a continuous phase, andmay also contain a dispersant such as a surfactant, if required. Thedispersion of the water-insoluble polymer particles B also acts as afixing emulsion for fixing ink droplets ejected from ink-jet nozzles ona recording medium and improving optical density, gloss and rub fastnessof the printed images or characters.

The water-insoluble polymer particles B may also contain a colorant suchas pigments and dyes, but preferably contain no colorant from theviewpoint of improving storage stability and ejection property of thewater-based ink.

The content of the water-insoluble polymer particles B in the dispersioncontaining the water-insoluble polymer particles B is preferably notless than 10% by mass, more preferably not less than 20% by mass andeven more preferably not less than 30% by mass, and is also preferablynot more than 70% by mass, more preferably not more than 60% by mass andeven more preferably not more than 55% by mass from the viewpoint ofenhancing dispersion stability of the water-insoluble polymer particlesB and for convenience of formulation of the ink.

The average particle size of the water-insoluble polymer particles B inthe dispersion containing the water-insoluble polymer particles B or thewater-based ink is preferably not less than 10 nm, more preferably notless than 30 nm and even more preferably not less than 50 nm, and isalso preferably not more than 300 nm, more preferably not more than 150nm and even more preferably not more than 100 nm from the viewpoint ofenhancing storage stability of the water-based ink and improving opticaldensity and gloss of the ink when printed on a low-water absorbingrecording medium.

Meanwhile, the average particle size of the water-insoluble polymerparticles B may be measured by the method described in Examples below.

Examples of the resin components of the water-insoluble polymerparticles B include an acrylic resin, a styrene-based resin, aurethane-based resin, a polyester-based resin, a styrene-acryl-basedresin, a butadiene-based resin, a styrene-butadiene-based resin, a vinylchloride-based resin, a vinyl acetate-based resin and an acrylicsilicone-based resin. Among these resin components, preferred is anacrylic resin from the viewpoint of promoting drying of the ink on thesurface of paper and improving rub fastness of the printed images orcharacters when printed on a low-water absorbing recording medium.

In addition, the water-insoluble polymer particles B used in the presentinvention are preferably used in the form of a dispersion containing thewater-insoluble polymer particles B from the viewpoint of improvingproductivity of the water-based ink.

The water-insoluble polymer particles B may be either an appropriatelysynthesized product or a commercially available product.

Examples of the commercially available product of the water-insolublepolymer particles B include acrylic resins such as “Neocryl A1127”(anionic self-crosslinked aqueous acrylic resin available from DSMNeoResins, Inc.), “JONCRYL 390” (available from BASF Japan, Ltd.),urethane resins such as “WBR-2018” and “WBR-2000U” (both available fromTaisei Fine Chemical Co., Ltd.), styrene-butadiene resins such as“SR-100” and “SR102” (both available from Nippon A & L Inc.),styrene-acrylic resins such as “JONCRYL 7100”, “JONCRYL 737” and“JONCRYL 538” (all available from BASF Japan, Ltd.), and vinylchloride-based resins such as “VINYBLAN 701” (Nissin Chemical IndustryCo., Ltd.).

The mass ratio of the pigment to the water-insoluble polymer particles B[pigment/water-insoluble polymer particles B] is preferably not lessthan 100/300, more preferably not less than 100/200 and even morepreferably not less than 100/150, and is also preferably not more than100/25, more preferably not more than 100/50 and even more preferablynot more than 100/70 from the viewpoint of promoting drying of printedimages or characters and improving rub fastness, optical density andgloss of the printed images or characters when printed on a low-waterabsorbing recording medium. Thus, the mass ratio of the pigment to thewater-insoluble polymer particles B [pigment/water-insoluble polymerparticles B] is preferably from 100/25 to 100/300, more preferably from100/50 to 100/200 and even more preferably from 100/70 to 100/150.

The content of the water-insoluble polymer particles B in thewater-based ink for ink-jet printing used in the present invention ispreferably not less than 1.0% by mass, more preferably not less than2.0% by mass and even more preferably not less than 3.0% by mass fromthe viewpoint of improving rub fastness of printed images or characterswhen printed on a low-water absorbing recording medium, and is alsopreferably not more than 10% by mass, more preferably not more than 8.0%by mass and even more preferably not more than 6.0% by mass from theviewpoint of reducing the viscosity of the ink upon evaporating thesolvent therefrom, increasing the dot size of the ink and enhancingoptical density and gloss of the ink when printed on a low-waterabsorbing recording medium as well as from the viewpoint of improvingthe storage stability and ejection property of the water-based ink.

[Organic Solvent C]

The water-based ink for ink-jet printing as used in the presentinvention contains the organic solvent C from the viewpoint of enhancingthe ejection property of the water-based ink. The organic solvent Ccontains one or more organic solvents having a boiling point of 90° C.or higher, and has an average boiling point boiling point of 250° C. orlower as a weighted mean value of boiling points that are weighted bythe content (% by mass) of respective organic solvents in the organicsolvent C. When using two or more organic solvents as the organicsolvent C, a plurality of organic solvents that have different boilingpoints from each other are preferably used.

The average boiling point boiling point of the organic solvent C as theweighted mean value is preferably not lower than 150° C. and morepreferably not lower than 180° C. from the viewpoint of preventingdry-out of the ink in ink-jet nozzles, and is also preferably not higherthan 240° C., more preferably not higher than 230° C. and even morepreferably not higher than 220° C. from the viewpoint of promotingdrying of printed images or characters and improving rub fastness of theprinted images or characters when printed on a low-water absorbingrecording medium.

As the boiling point of an organic solvent is lowered, the saturatedvapor pressure and evaporation rate of the organic solvent as measuredat a predetermined temperature are increased. In addition, as aproportion of the organic solvent having a high evaporation rate in amixed organic solvent as measured at a predetermined temperature isincreased, the evaporation rate of the mixed organic solvent as measuredat the predetermined temperature is increased. Therefore, the weightedmean value of the boiling point of the organic solvent C is regarded asan index of the evaporation rate of the mixed solvent.

Meanwhile, the weighted mean value of the boiling point of the organicsolvent C having, for example, a glycerin content of 10% by mass and apropylene glycol content of 30% by mass is 214° C. as the valuecalculated from the following formula.

[glycerin content (% by mass)×boiling point of glycerin (290°C.)]+[propylene glycol content (% by mass)×boiling point of propyleneglycol (188° C.)]/[glycerin content (% by mass)+propylene glycol content(% by mass)]=[[0.1×290° C.]+[0.3×188°]]/[0.1+0.3]=214° C.

Examples of the compound used as the organic solvent C includepolyhydric alcohols, polyhydric alcohol alkyl ethers,nitrogen-containing heterocyclic compounds, amides, amines andsulfur-containing compounds. Among these compounds, from the viewpointof improving the storage stability and ejection property of thewater-based ink, preferred is at least one compound selected from thegroup consisting of polyhydric alcohols and polyhydric alcohol alkylethers, and more preferred are polyhydric alcohols. The polyhydricalcohols may be in the form of a mixed alcohol containing a plurality ofcompounds belonging to the concept of polyhydric alcohols, and thepolyhydric alcohol alkyl ethers may also be in the form of a mixed ethercontaining a plurality of compounds belonging to the concept ofpolyhydric alcohol alkyl ethers.

The content of at least one compound selected from the group consistingof polyhydric alcohols and polyhydric alcohol alkyl ethers in theorganic solvent C is preferably not less than 80% by mass, morepreferably not less than 90% by mass, even more preferably not less than95% by mass, further even more preferably substantially 100% by mass,and even more preferably 100% by mass.

Examples of the polyhydric alcohols include ethylene glycol (b.p. 197°C.), diethylene glycol (b.p. 244° C.), triethylene glycol (b.p. 285°C.), polyethylene glycol, propylene glycol (b.p. 188° C.), dipropyleneglycol (b.p. 232° C.), tripropylene glycol (b.p. 273° C.), polypropyleneglycol, 1,3-propanediol (b.p. 210° C.), 1,3-butanediol (b.p. 208° C.),1,4-butanediol (b.p. 230° C.), 3-methyl-1,3-butanediol (b.p. 203° C.),1,5-pentanediol (b.p. 242° C.), 1,6-hexanediol (b.p. 250° C.),2-methyl-2,4-pentanediol (b.p. 196° C.), glycerin (b.p. 290° C.),1,2,6-hexanetriol (b.p. 178° C.), 1,2,4-butanetriol (b.p. 190° C.),1,2,3-butanetriol (b.p. 175° C.) and petriol(3-methyl-1,3,5-pentanetriol) (b.p. 216° C.). Triethylene glycol,tripropylene glycol and glycerin may be used in combination with acompound having a boiling point lower than 250° C. Among thesepolyhydric alcohols, from the viewpoint of attaining excellent spread ofthe dot size and improving optical density, gloss and rub fastness ofthe ink when printed on a low-water absorbing recording medium,preferred are glycerin, propylene glycol and diethylene glycol.

Examples of the polyhydric alcohol alkyl ethers include ethylene glycolmonoethyl ether (b.p. 135° C.), ethylene glycol monobutyl ether (b.p.171° C.), diethylene glycol monomethyl ether (b.p. 194° C.), diethyleneglycol monoethyl ether (b.p. 202° C.), diethylene glycol monobutyl ether(b.p. 230° C.), triethylene glycol monomethyl ether (b.p. 122° C.),triethylene glycol monobutyl ether (b.p. 276° C.), triethylene glycolmonoisobutyl ether (b.p. 160° C.), tetraethylene glycol monomethyl ether(b.p. 158° C.), propylene glycol monoethyl ether (b.p. 133° C.),dipropylene glycol monobutyl ether (b.p. 227° C.), dipropylene glycolmonomethyl ether (b.p. 90° C.), tripropylene glycol monomethyl ether(b.p. 100° C.) and tripropylene glycol monobutyl ether. Triethyleneglycol monobutyl ether may be used in combination with a compound havinga boiling point lower than 250° C. Among these polyhydric alcohol alkylethers, from the viewpoint of attaining excellent spread of the dot sizeand improving optical density, gloss and rub fastness of the ink whenprinted on a low-water absorbing recording medium, preferred isdipropylene glycol monomethyl ether.

Examples of the nitrogen-containing heterocyclic compounds includeN-methyl-2-pyrrolidone (b.p. 202° C.), 2-pyrrolidone (b.p. 245° C.),1,3-dimethyl imidazoline (b.p. 220° C.) and ε-caprolactam (b.p. 136°C.).

Examples of the amides include formamide (b.p. 210° C.), N-methylformamide (b.p. 199° C.) and N,N-dimethyl formamide (b.p. 153° C.).

Examples of the amines include monoethanolamine (b.p. 170° C.),diethanolamine (b.p. 217° C.), triethanolamine (b.p. 208° C.) andtriethylamine (b.p. 90° C.).

Examples of the sulfur-containing compounds include dimethyl sulfoxide(b.p. 189° C.), sulfolane (b.p. 285° C.) and thiodiglycol (b.p. 282°C.). Sulfolane and thiodiglycol may be used in combination with acompound having a boiling point lower than 250° C.

Among these compounds, from the viewpoint of attaining excellent spreadof the dot size and improving optical density, gloss and rub fastness ofthe ink when printed on a low-water absorbing recording medium as wellas from the viewpoint of enhancing ejection property of the ink,preferred is a combination of two or more kinds of polyhydric alcohols,combination of two or more kinds of polyhydric alcohol alkyl ethers andcombination of at least one polyhydric alcohol and at least onepolyhydric alcohol alkyl ether; more preferred is a combination of twoor more kinds of polyhydric alcohols or combination of at least onepolyhydric alcohol and at least one polyhydric alcohol alkyl ether; andeven more preferred is a combination of at least one compound selectedfrom the group consisting of propylene glycol, diethylene glycol anddipropylene glycol monomethyl ether, and glycerin.

The mass ratio of the organic solvent (X) as the at least one compoundselected from the group consisting of propylene glycol, diethyleneglycol and dipropylene glycol monomethyl ether to glycerin [organicsolvent (X)/glycerin] is preferably not less than 0.5, more preferablynot less than 1.0 and even more preferably not less than 2.0 from theviewpoint of promoting drying of printed images or characters andimproving rub fastness of the printed images or characters when printedon a low-water absorbing recording medium, and is also preferably notmore than 20, more preferably not more than 17 and even more preferablynot more than 13 from the viewpoint of enhancing ejection property ofthe ink.

The content of the organic solvent C in the water-based ink ispreferably not less than 20% by mass, more preferably not less than 30%by mass and even more preferably not less than 35% by mass from theviewpoint of enhancing ejection property of the ink, and is alsopreferably not more than 60% by mass, more preferably not more than 50%by mass and even more preferably not more than 45% by mass from theviewpoint of enhancing storage stability of the water-based ink, as wellas from the viewpoint of promoting drying of printed images orcharacters and improving rub fastness of the printed images orcharacters when printed on a low-water absorbing recording medium.

The total content of propylene glycol, diethylene glycol, dipropyleneglycol monomethyl ether and glycerin in the water-based ink ispreferably not less than 20% by mass, more preferably not less than 30%by mass and even more preferably not less than 35% by mass from theviewpoint of enhancing ejection property of the ink, and is alsopreferably not more than 60% by mass, more preferably not more than 50%by mass and even more preferably not more than 45% by mass from theviewpoint of enhancing the storage stability of the water-based ink, aswell as from the viewpoint of promoting drying of printed images orcharacters and improving rub fastness of the printed images orcharacters when printed on a low-water absorbing recording medium.

The total content of propylene glycol, diethylene glycol, dipropyleneglycol monomethyl ether and glycerin in the organic solvent C ispreferably not less than 80% by mass, more preferably not less than 90%by mass, even more preferably not less than 95% by mass and further evenmore preferably substantially 100% by mass from the viewpoint ofenhancing the ejection property of the ink, as well as from theviewpoint of promoting drying of printed images or characters andimproving rub fastness of the printed images or characters when printedon a low-water absorbing recording medium.

The total content of propylene glycol and diethylene glycol in thewater-based ink is preferably not less than 15% by mass, more preferablynot less than 20% by mass and even more preferably not less than 25% bymass from the viewpoint of attaining excellent spread of the dot sizeand improving optical density, gloss and rub fastness of the ink whenprinted on a low-water absorbing recording medium, and is alsopreferably not more than 55% by mass, more preferably not more than 45%by mass and even more preferably not more than 40% by mass from the sameviewpoints as described above.

The content of dipropylene glycol monomethyl ether in the water-basedink is preferably not less than 1% by mass, more preferably not lessthan 1.5% by mass and even more preferably not less than 2% by mass fromthe viewpoint of attaining excellent spread of the dot size andimproving optical density, gloss and rub fastness of the ink whenprinted on a low-water absorbing recording medium, and is alsopreferably not more than 30% by mass, more preferably not more than 10%by mass and even more preferably not more than 5% by mass from the sameviewpoints as described above.

The content of glycerin in the water-based ink is preferably not lessthan 1% by mass, more preferably not less than 3% by mass and even morepreferably not less than 5% by mass from the viewpoint of enhancingejection property of the ink, and is also preferably not more than 20%by mass, more preferably not more than 15% by mass and even morepreferably not more than 10% by mass from the viewpoint of attainingexcellent spread of the dot size and improving optical density, glossand rub fastness of the ink when printed on a low-water absorbingrecording medium.

[Water]

The content of water in the water-based ink for ink-jet printing ispreferably not less than 20% by mass, more preferably not less than 30%by mass and even more preferably not less than 40% by mass from theviewpoint of attaining excellent spread of the dot size and improvingoptical density, gloss and rub fastness of the ink when printed on alow-water absorbing recording medium as well as from the viewpoint ofenhancing storage stability of the water-based ink, and is alsopreferably not more than 70% by mass, more preferably not more than 65%by mass and even more preferably not more than 60% by mass from theviewpoint of enhancing ejection property of the ink.

[Other Components]

The water-based ink for ink-jet printing according to the presentinvention may also contain various ordinary additives such as a wettingagent, a penetrant, a dispersant, a surfactant, a viscosity controller,a defoaming agent, an antiseptic agent, a mildew-proof agent and a rustpreventive.

[Process for Producing Water-Based Ink for Ink-Jet Printing]

The water-based ink of ink-jet printing may be produced by mixing awater dispersion of the pigment-containing water-insoluble polymerparticles A, the water-insoluble polymer particles B, water and theorganic solvent C, if required, together with a surfactant, etc., witheach other while stirring. Upon the mixing, the water-insoluble polymerparticles A and the water-insoluble polymer particles B are preferablyadded and mixed in this order from the viewpoint of suppressingaggregation of these particles.

[Properties of Ink]

The viscosity of the water-based ink for ink-jet printing as measured at32° C. is preferably not less than 2.0 mPa·s; more preferably not lessthan 3.0 mPa·s and even more preferably not less than 5.0 mPa·s from theviewpoint of enhancing ejection property of the water-based ink, and isalso preferably not more than 12 mPa·s, more preferably not more than9.0 mPa·s and even more preferably not more than 7.0 mPa·s from theviewpoint of enhancing the storage stability and ejection property ofthe water-based ink.

Meanwhile, the viscosity of the water-based ink at 32° C. may bemeasured by the method described in Examples below.

The pH value of the water-based ink for ink-jet printing is preferablynot less than 7.0, more preferably not less than 8.0 and even morepreferably not less than 8.5 from the viewpoint of enhancing the storagestability and ejection property of the water-based ink and from theviewpoint of attaining excellent spread of the dot size and improvingoptical density, gloss and rub fastness of the ink when printed on alow-water absorbing recording medium, and is also preferably not morethan 11.0, more preferably not more than 10.0 and even more preferably9.5 from the viewpoint of improving resistance of members to the ink andsuppressing skin irritation. Meanwhile, the pH value of the water-basedink may be measured by the method described in Examples below.

<Recording Medium for Ink-Jet Printing> [Water Absorption]

The water absorption of a recording medium for ink-jet printing asmeasured in a pure water contact time of 100 ms is not less than 0 g/m²and not more than 10 g/m². More specifically, the water absorption ofthe recording medium for ink-jet printing as measured in a pure watercontact time of 100 ms is preferably not less than 1.0 g/m², morepreferably not less than 2.0 g/m², even more preferably not less than3.0 g/m² and further even more preferably not less than 4.0 g/m² fromthe viewpoint of promoting drying of printed images or characters andenhancing rub fastness of the printed images or characters, and is alsopreferably not more than 8.0 g/m², more preferably not more than 7.0g/m², even more preferably not more than 6.0 g/m² and further even morepreferably not more than 5.5 g/m² from the viewpoint of enhancingoptical density and gloss of the ink.

Meanwhile, the water absorption of the recording medium for ink-jetprinting as measured in a pure water contact time of 100 ms may bemeasured by the method described in Examples below.

[Properties]

The 60° gloss of the recording medium is preferably not less than 5,more preferably not less than 20 and even more preferably not less than30, and is also preferably not more than 200, from the viewpoint ofenhancing gloss and visibility of printed images or characters. The 60°gloss of the recording medium may be measured by a gloss meter “HANDYGLOSSMETER, Model No.: PG-1M” available from Nippon Denshoku IndustriesCo., Ltd.

The recording medium for ink-jet printing used in the present inventionmay be a coated paper or a film.

Examples of the coated paper include “OK Topcoat+” (available from OjiPaper Co., Ltd.; basis weight: 104.7 g/m²; 60° gloss: 49.0; waterabsorption as measured in a pure water contact time of 100 ms(hereinafter defined in the same way): 4.9 g/m²), a multi-color foamgloss coated paper (available from Oji Paper Co., Ltd.; basis weight:104.7 g/m²; 60° gloss: 36.8; water absorption: 5.2 g/m²), “UPM FinesseGloss” (available from UPM; basis weight: 115 g/m²; 60° gloss: 27.0;water absorption: 3.1 g/m²), “UPM Finesse Matt” (available from UPM;basis weight: 115 g/m²; 60° gloss: 5.6; water absorption: 4.4 g/m²),“TerraPress Silk” (available from Stora Enso; basis weight: 80 g/m²; 60°gloss: 6.0; water absorption: 4.1 g/m²), and “LumiArt” (available fromStora Enso; basis weight: 90 g/m²; 60° gloss: 26.3).

Examples of the film include a polyester film, a vinyl chloride film, apolypropylene film, a polyethylene film and a nylon film. These filmsmay be subjected to surface treatments such as corona treatment, etc.,if required.

Examples of the generally available films include “LUMIRROR T60”(available from Toray Industries Inc.; polyethylene terephthalate;thickness: 125 μm; 60° gloss: 189.1; water absorption: 2.3 g/m²),“PVC80B P” (available from Lintec Corp.; polyvinyl chloride; 60° gloss:58.8; water absorption: 1.4 g/m²), “KINATH KEE 70CA” (available fromLintec Corp.; polyethylene), “YUPO SG90 PAT1” (available from LintecCorp.; polypropylene), and “BONYL RX” (available from Kohjin Film &Chemical Co., Ltd.; nylon).

<Ink-Jet Printing Method>

In the ink-jet printing method according to the present invention, theabove water-based ink for ink-jet printing is loaded in an ink-jetprinting apparatus equipped with an ink ejecting means, and ejected ontothe recording medium for ink-jet printing to print images or charactersthereon.

The ink-jet printing method may also include the step of drying theimages or characters printed on the recording medium for ink-jetprinting.

The ink ejecting means may include a thermal-type or piezoelectric-typeink-jet head for ejecting the ink therethrough. Among these methods, themethod of ejecting the ink using a piezoelectric-type ink-jet head toprint the images or characters is preferably used in the presentinvention.

<Use of Water-Based Ink for Ink-Jet Printing for Printing>

When using the water-based ink for ink-jet printing according to thepresent invention for printing, the water-based ink for ink-jet printingis used for printing images or characters on a recording medium having awater absorption of not less than 0 g/m² and not more than 10 g/m² asmeasured in a pure water contact time of 100 ms.

With respect to the aforementioned embodiments, the present inventionfurther provides the following aspects relating to the ink-jet printingmethods and water-based inks for ink-jet printing.

<1> An ink-jet printing method for printing images or characters on arecording medium using a water-based ink for ink-jet printing, in whichthe water-based ink includes pigment-containing water-insoluble polymerparticles A, water-insoluble polymer particles B, an organic solvent Cand water;

the water-insoluble polymer particles A includes a water-insolublepolymer (a) containing a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the aboveformula (1), the constitutional unit derived from the hydrophilicmonomer (a-3) being present in an amount of not less than 13% by massand not more than 45% by mass on the basis of whole constitutional unitsin the water-insoluble polymer (a);

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C; and

the recording medium has a water absorption of not less than 0 g/m² andnot more than 10 g/m² as measured in a pure water contact time of 100ms.

<2> The ink-jet printing method as described in the aspect <1>, whereinin the formula (1), m is preferably not less than 9, more preferably notless than 12, even more preferably not less than 16 and further evenmore preferably not less than 20, and is also preferably not more than90, more preferably not more than 70, even more preferably not more than55 and further even more preferably not more than 25.<3> The ink-jet printing method as described in the aspect <1> or <2>,wherein in the formula (1), m is preferably from 12 to 90, morepreferably from 16 to 70, even more preferably from 20 to 55 and furthereven more preferably from 20 to 25.<4> The ink-jet printing method as described in any one of the aspects<1> to <3>, wherein in the formula (1), R² is preferably a hydrogen atomor an alkyl group having not less than 1 an not more than 20 carbonatoms, preferably an alkyl group having not less than 1 and not morethan 3 carbon atoms and more preferably a methyl group.<5> The ink-jet printing method as described in any one of the aspects<1> to <4>, wherein in the formula (1), R¹ is preferably a methyl group.<6> The ink-jet printing method as described in any one of the aspects<1> to <5>, wherein the monomer (a-3) is preferably at least onecompound selected from the group consisting of polyethylene glycolmono(meth)acrylate, methoxy polyethylene glycol mono(meth)acrylate,octoxy polyethylene glycol mono(meth)acrylate and stearoyloxypolyethylene glycol mono(meth)acrylate, and more preferably methoxypolyethylene glycol mono(meth)acrylate.<7> The ink-jet printing method as described in any one of the aspects<1> to <6>, wherein the monomer (a-1) is preferably a carboxylic acidmonomer, more preferably acrylic acid or methacrylic acid, and even morepreferably methacrylic acid.<8> The ink-jet printing method as described in any one of the aspects<1> to <7>, wherein the monomer (a-2) is preferably at least one monomerselected from the group consisting of a styrene-based monomer, anaromatic group-containing (meth)acrylate and a styrene-basedmacromonomer, more preferably at least one monomer selected from thegroup consisting of an aromatic group-containing (meth)acrylate and astyrene-based macromonomer, even more preferably combination of anaromatic group-containing (meth)acrylate and a styrene-basedmacromonomer, and further even more preferably combination ofbenzyl(meth)acrylate and a styrene-based macromonomer.<9> The ink-jet printing method as described in any, one of the aspects<1> to <8>, wherein upon production of the water-insoluble polymer (a),a content of the monomer (a-3) in a monomer mixture containing themonomers (a-1), (a-2) and (a-3) or a content of a constitutional unitderived from the monomer (a-3) in the water-insoluble polymer (a) ispreferably not less than 15% by mass and more preferably not less than18% by mass, and is also preferably not more than 35% by mass and morepreferably not more than 25% by mass, and is further preferably from 15to 35% by mass and more preferably from 18 to 25% by mass.<10> The ink-jet printing method as described in any one of the aspects<1> to <9>, wherein upon production of the water-insoluble polymer (a),a content of the monomer (a-1) in a monomer mixture containing themonomers (a-1), (a-2) and (a-3), or a content of a constitutional unitderived from the monomer (a-1) in the water-insoluble polymer (a) ispreferably not less than 3% by mass, more preferably not less than 5% bymass and even more preferably not less than 8% by mass, and is alsopreferably not more than 40% by mass, more preferably not more than 25%by mass and even more preferably not more than 20% by mass.<11> The ink-jet printing method as described in any one of the aspects<1> to <10>, wherein upon production of the water-insoluble polymer (a),a content of the monomer (a-2) in a monomer mixture containing themonomers (a-1), (a-2) and (a-3) or a content of a constitutional unitderived from the monomer (a-2) in the water-insoluble polymer (a) ispreferably not less than 20% by mass, more preferably not less than 40%by mass and even more preferably not less than 55% by mass, and is alsopreferably not more than 80% by mass, more preferably not more than 75%by mass and even more preferably not more than 70% by mass.<12> The ink-jet printing method as described in any one of the aspects<1> to <11>, wherein upon production of the water-insoluble polymer (a),a content of the styrene-based macromonomer in a monomer mixturecontaining the monomers (a-1), (a-2) and (a-3) or a content of aconstitutional unit derived from the styrene-based macromonomer in thewater-insoluble polymer (a) is preferably not less than 5% by mass andmore preferably not less than 8% by mass, and is also preferably notmore than 15% by mass and more preferably not more than 12% by mass.<13> The ink-jet printing method as described in any one of the aspects<1> to <12>, wherein a mass ratio of the component (a-1) to a sum of thecomponent (a-2) and the component (a-3) [component (a-1)/[component(a-2)+component (a-3)]] is preferably not less than 0.03, morepreferably not less than 0.05 and even more preferably not less than0.10, and is also preferably not more than 0.50, more preferably notmore than 0.30 and even more preferably not more than 0.20.<14> The ink-jet printing method as described in any one of the aspects<1> to <13>, wherein the average boiling point of the organic solvent Cas a weighted mean value is preferably not lower than 150° C. and morepreferably not lower than 180° C., and is also preferably not higherthan 240° C., more preferably not higher than 230° C. and even morepreferably not higher than 220° C.<15> The ink-jet printing method as described in any one of the aspects<1> to <14>, wherein the organic solvent C is preferably at least onecompound selected from the group consisting of a polyhydric alcohol anda polyhydric alcohol alkyl ether, more preferably combination of two ormore kinds of polyhydric alcohols, combination of two or more kinds ofpolyhydric alcohol alkyl ethers or combination of at least onepolyhydric alcohol and at least one polyhydric alcohol alkyl ether, evenmore preferably combination of two or more kinds of polyhydric alcoholsor combination of at least one polyhydric alcohol and at least onepolyhydric alcohol alkyl ether, and further even more preferablycombination of at least one compound selected from the group consistingof propylene glycol, diethylene glycol and dipropylene glycol monomethylether, and glycerin.<16> The ink-jet printing method as described in any one of the aspects<1> to <15>, wherein a content of the at least one compound selectedfrom the group consisting of a polyhydric alcohol and a polyhydricalcohol alkyl ether in the organic solvent C is preferably not less than80% by mass, more preferably not less than 90% by mass, even morepreferably not less than 95% by mass, further even more preferablysubstantially 100% by mass, and even more preferably 100% by mass.<17> The ink-jet printing method as described in any one of the aspects<1> to <16>, wherein a mass ratio of at least one organic solvent (X)selected from the group consisting of propylene glycol, diethyleneglycol and dipropylene glycol monomethyl ether to glycerin [organicsolvent (X)/glycerin] is preferably not less than 0.5, more preferablynot less than 1.0 and even more preferably not less than 2.0, and isalso preferably not more than 20, more preferably not more than 17 andeven more preferably not more than 13.<18> The ink-jet printing method as described in any one of the aspects<1> to <17>, wherein a content of the pigment-containing water-insolublepolymer particles A in the water-based ink is preferably not less than1% by mass, more preferably not less than 3% by mass and even morepreferably not less than 4% by mass, and is also preferably not morethan 20% by mass, more preferably not more than 10% by mass and evenmore preferably not more than 8% by mass.<19> The ink-jet printing method as described in any one of the aspects<1> to <18>, wherein a content of the pigment in the water-based ink ispreferably not less than 1% by mass, more preferably not less than 2% bymass and even more preferably not less than 3% by mass, and is alsopreferably not more than 15% by mass, more preferably not more than 10%by mass and even more preferably not more than 6% by mass.<20> The ink-jet printing method as described in any one of the aspects<1> to <19>, wherein a content of the water-insoluble polymer (a) in thewater-based ink is preferably not less than 0.5% by mass, morepreferably not less than 0.8% by mass and even more preferably not lessthan 1% by mass, and is also preferably not more than 6% by mass, morepreferably not more than 3% by mass and even more preferably not morethan 2% by mass.<21> The ink-jet printing method as described in any one of the aspects<1> to <20>, wherein a content of the water-insoluble polymer particlesB in the water-based ink is preferably not less than 1.0% by mass, morepreferably not less than 2.0% by mass and even more preferably not lessthan 3.0% by mass, and is also preferably not more than 10% by mass,more preferably not more than 8.0% by mass and even more preferably notmore than 6.0% by mass.<22> The ink-jet printing method as described in any one of the aspects<1> to <21>, wherein a mass ratio of the pigment to the water-insolublepolymer particles B [pigment/water-insoluble polymer particles B] ispreferably not less than 100/300, more preferably not less than 100/200and even more preferably not less than 100/150, and is also preferablynot more than 100/25, more preferably not more than 100/50 and even morepreferably not more than 100/70, and further is preferably from 100/25to 100/300, more preferably from 100/50 to 100/200 and even morepreferably from 100/70 to 100/150.<23> The ink-jet printing method as described in any one of the aspects<1> to <22>, wherein a content of the organic solvent C in thewater-based ink is preferably not less than 20% by mass, more preferablynot less than 30% by mass and even more preferably not less than 35% bymass, and is also preferably not more than 60% by mass, more preferablynot more than 50% by mass and even more preferably not more than 45% bymass.<24> The ink-jet printing method as described in any one of the aspects<1> to <23>, wherein a content of glycerin in the water-based ink ispreferably not less than 1% by mass, more preferably not less than 3% bymass and even more preferably not less than 5% by mass, and is alsopreferably not more than 20% by mass, more preferably not more than 15%by mass and even more preferably not more than 10% by mass.<25> The ink-jet printing method as described in any one of the aspects<1> to <24>, wherein a total content of propylene glycol and diethyleneglycol in the water-based ink is preferably not less than 15% by mass,more preferably not less than 20% by mass and even more preferably notless than 25% by mass, and is also preferably not more than 55% by mass,more preferably not more than 45% by mass and even more preferably notmore than 40% by mass.<26> The ink-jet printing method as described in any one of the aspects<1> to <25>, wherein a total content of propylene glycol, diethyleneglycol, dipropylene glycol monomethyl ether and glycerin in thewater-based ink is preferably not less than 30% by mass and morepreferably not less than 35% by mass, and is also preferably not morethan 60% by mass, more preferably not more than 50% by mass and evenmore preferably not more than 45% by mass.<27> The ink-jet printing method as described in any one of the aspects<1> to <26>, wherein a water absorption of the recording medium forink-jet printing as measured in a pure water contact time of 100 ms ispreferably not less than 1.0 g/m², more preferably not less than 2.0g/m², even more preferably not less than 3.0 g/m² and further even morepreferably not less than 4.0 g/m², and is also preferably not more than8.0 g/m², more preferably not more than 7.0 g/m², even more preferablynot more than 6.0 g/m² and further even more preferably not more than5.5 g/m².<28> The ink-jet printing method as described in any one of the aspects<1> to <27>, wherein the pigment-containing water-insoluble polymerparticles A in the form of a pigment water dispersion are compounded inthe water-based ink for ink-jet printing.<29> The ink-jet printing method as described in any one of the aspects<1> to <28>, wherein the pigment water dispersion of thepigment-containing water-insoluble polymer particles A is produced bythe following steps (1) and (2):

Step (1): subjecting a mixture of the water-insoluble polymer (a), theorganic solvent (b), the pigment and water to dispersion treatment toobtain a dispersion treatment product; and

Step (2): removing the organic solvent (b) from the dispersion treatmentproduct obtained in the step (1) to obtain the pigment water dispersion.

<30> The ink-jet printing method as described in any one of the aspects<1> to <29>, wherein a weight-average molecular weight of thewater-insoluble polymer (a) is preferably not less than 10,000, morepreferably not less than 15,000 and even more preferably not less than20,000, and is also preferably not more than 150,000, more preferablynot more than 100,000 and even more preferably not more than 60,000.<31> The ink-jet printing method as described in any one of the aspects<1> to <30>, wherein an average particle size of the pigment-containingwater-insoluble polymer particles A in the pigment water dispersion orthe water-based ink is preferably not less than 40 nm, more preferablynot less than 60 nm and even more preferably not less than 75 nm, and isalso preferably not more than 150 nm, more preferably not more than 120nm, even more preferably not more than 110 nm and further even morepreferably not more than 100 nm.<32> The ink-jet printing method as described in any one of the aspects<1> to <31>, wherein an average particle size of the water-insolublepolymer particles B in the dispersion containing the water-insolublepolymer particles B or the water-based ink is preferably not less than10 nm, more preferably not less than 30 nm and even more preferably notless than 50 nm, and is also preferably not more than 300 nm, morepreferably not more than 150 nm and even more preferably not more than100 nm.<33> The ink-jet printing method as described in any one of the aspects<1> to <32>, wherein the water-based ink for ink-jet printing is loadedto an ink-jet printing apparatus equipped with an ink ejecting means,and ejected onto the recording medium for ink-jet printing to printimages or characters thereon.<34> The ink-jet printing method as described in the aspect <33>,wherein the ink ejecting means includes a thermal-type orpiezoelectric-type ink-jet head, preferably a piezoelectric-type ink-jethead, for ejecting the ink therethrough.<35> The ink-jet printing method as described in any one of the aspects<1> to <34>, comprising the steps of printing images or characters onthe recording medium for ink-jet printing and then drying the printedimages or characters.<36> A water-based ink for ink-jet printing, includingpigment-containing water-insoluble polymer particles A, water-insolublepolymer particles B, an organic solvent C and water, in which

the water-insoluble polymer particles A include a water-insolublepolymer (a) containing a constitutional unit derived from an ionicmonomer (a-1), a constitutional unit derived from an aromaticring-containing hydrophobic monomer (a-2) and a constitutional unitderived from a hydrophilic monomer (a-3) represented by the aboveformula (1), the constitutional unit derived from the hydrophilicmonomer (a-3) being present in an amount of not less than 13% by massand not more than 45% by mass on the basis of whole constitutional unitsin the water-insoluble polymer (a); and

the organic solvent C includes one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by the content (% by mass) of respective organic solvents inthe organic solvent C.

<37> The water-based ink for ink-jet printing as described in the aspect<36>, wherein the organic solvent C includes at least one compoundselected from the group consisting of a polyhydric alcohol and apolyhydric alcohol alkyl ether.<38> The water-based ink for ink-jet printing as described in the aspect<36> or <37>, wherein a content of the at least one compound selectedfrom the group consisting of a polyhydric alcohol and a polyhydricalcohol alkyl ether in the organic solvent C is not less than 80% bymass.<39> The water-based ink for ink-jet printing as described in any one ofthe aspects <36> to <38>, wherein the organic solvent C includes atleast one compound selected from the group consisting of propyleneglycol, diethylene glycol and dipropylene glycol monomethyl ether, andglycerin.<40> The water-based ink for ink-jet printing as described in any one ofthe aspects <36> to <39>, wherein the aromatic ring-containinghydrophobic monomer (a-2) includes at least one monomer selected fromthe group consisting of a styrene-based monomer, an aromaticgroup-containing (meth)acrylate and a styrene-based macromonomer.

EXAMPLES

In the following production examples, examples and comparative examples,the “part(s)” and “%” indicate “part(s) by mass” and “% by mass”,respectively, unless otherwise specified.

(1) Measurement of Weight-Average Molecular Weight of Polymer

The weight-average molecular weight of the polymer was measured by gelchromatographic method [GPC apparatus: “HLC-8120GPC” available fromTosoh Corp.; column: “TSK-GEL α-M”×2 available from Tosoh Corp.; flowrate: 1 mL/min)] using N,N-dimethyl formamide in which phosphoric acidand lithium bromide were dissolved in amounts of 60 mmol/L and 50mmol/L, respectively, as an eluent, and using a polystyrene as areference standard substance.

(2) Measurement of Average Particle Sizes of Pigment-ContainingWater-Insoluble Polymer Particles A and Water-Insoluble PolymerParticles B

The particles were subjected to cumulant analysis using a laser particleanalyzing system “ELS-8000” (available from Otsuka Electrics Co., Ltd.)to measure the average particle size thereof. The measurement wasconducted at a temperature of 25° C., an angle between incident lightand detector of 90° and a cumulative frequency of 100 times, and arefractive index of water (1.333) was input to the analyzing system as arefractive index of the dispersing medium. Further, the measurement wasconducted by adjusting the concentration of the dispersion to bemeasured to 5×10⁻³% by mass in terms of the solid content thereof.

Meanwhile, the water-insoluble polymer particles B (anionicself-crosslinked aqueous acrylic resin “Neocryl A1127”) used in Exampleshad an average particle size of 63.2 nm.

(3) Measurement of Solid Content of Pigment Water Dispersion

Ten grams (10.0 g) of sodium sulfate dried to constant weight in adesiccator were weighed and charged in a 30 mL polypropylene container(φ: 40 mm; height: 30 mm), and about 1.0 g of a sample was added to thecontainer. The content of the container were mixed and then accuratelyweighed. The resulting mixture was held in the container at 105° C. for2 h to remove volatile components therefrom and further allowed to standin a desiccator for 15 min to measure the mass thereof. The mass of thesample after removing the volatile components therefrom was regarded asa mass of solids therein. The solid content of the sample was calculatedby dividing the mass of the solids by the mass of the sample added.

(4) pH of Ink

The pH value of the ink at 25° C. was measured using a bench-top pHmeter “F-71” available from Horiba Ltd., equipped with a pH electrode“6337-10D” available from Horiba Ltd.

(5) Viscosity of Ink

The viscosity of the ink was measured at 32° C. using a E-typeviscometer “TV-25” (equipped with a standard cone rotor (1° 34′×R24);rotating speed: 50 rpm) available from Told Sangyo Co., Ltd.

(6) Water Absorption of Recording Medium as Measured in Pure WaterContact Time of 100 ms

Using an automatic scanning liquid absorptometer “KM500win” availablefrom Kumagai Riki Kogyo Co., Ltd., the amount of pure water transferredto a recording medium when contacted with pure water for 100 ms wasmeasured at 23° C. under a relative humidity of 50%. The thus measuredamount of pure water transferred to the recording medium was determinedas a water absorption of the recording medium as measured in a purewater contact time of 100 ms. The measuring conditions are shown below.

“Spiral Method”

Contact time: 0.010 to 1.0 (sec)

Pitch (mm): 7

Length Per Sampling (degree): 86.29

Start Radius (mm): 20

End Radius (mm): 60

Min Contact Time (ms): 10

Max Contact Time (ms): 1000

Sampling Pattern (1-50): 50

Number of Sampling Points (>0): 19

“Square Head”

Split Span (mm): 1

Split Width (mm): 5

The values of water absorption of an ordinary gloss coated paper “OKTopcoat+” (available from Oji Paper Co., Ltd.), a PET film “LUMIRRORT60” (available from Toray Industries Inc.; thickness: 125 μm) and aplain paper “4200” (available from Fuji Xerox Co., Ltd.) were 4.9 g/m²,2.3 g/m² and 14.0 g/m², respectively.

Production Examples I-1 to I-12 Production of Water-Insoluble PolymerSolutions 1 to 12

The monomers, solvent, polymerization initiator and chain transfer agentin each column “Initially Charged Monomer Solution” as shown in Tables 1and 2 were charged into a reaction vessel equipped with two droppingfunnels 1 and 2 and mixed with each other, and an inside atmosphere ofthe reaction vessel was replaced with a nitrogen gas, thereby obtainingan initially charged monomer solution.

On the other hand, the monomers, solvent, polymerization initiator andchain transfer agent in each column “Dropping Monomer Solution 1” asshown in Tables 1 and 2 were mixed with each other to obtain a droppingmonomer solution 1. The resulting dropping monomer solution 1 wascharged into the dropping funnel 1, and an inside atmosphere of thedropping funnel 1 was replaced with nitrogen gas.

In addition, the monomers, solvent, polymerization initiator and chaintransfer agent in each column “Dropping Monomer Solution 2” as shown inTables 1 and 2 were mixed with each other to obtain a dropping monomersolution 2. The resulting dropping monomer solution 2 was charged intothe dropping funnel 2, and the inside atmosphere of the dropping funnel2 was replaced with nitrogen gas.

In a nitrogen atmosphere, the initially charged monomer solution in thereaction vessel was held at 77° C. while stirring, and the droppingmonomer solution 1 in the dropping funnel 1 was gradually added dropwiseto the reaction vessel over 3 h. Next, the dropping monomer solution 2in the dropping funnel 2 was gradually added dropwise to the reactionvessel over 2 h. After completion of the dropwise addition, the mixedsolution in the reaction vessel was stirred at 77° C. for 0.5 h. Then, apolymerization initiator solution prepared by dissolving 1.1 parts ofthe above polymerization initiator2,2′-azobis(2,4-dimethylvaleronitrile) (“V-65” (tradename) availablefrom Wako Pure Chemical Industries, Ltd.) in 47.3 parts of methyl ethylketone was added to the mixed solution, and the resulting reactionsolution was aged at 77° C. for 0.5 h while stirring. The aboveprocedure including the preparation and addition of the polymerizationinitiator solution and the aging of the reaction solution was repeatedfive more times. Then, after maintaining the reaction solution in thereaction vessel at 80° C. for 1 h, about 200 parts of methyl ethylketone were added thereto to adjust the solid content of the reactionsolution to 45.2%, thereby obtaining a polymer solution.

The compositions and weight-average molecular weights of the thusobtained water-insoluble polymers 1 to 12 are shown in Table 3.

Meanwhile, the details of the monomers used herein were as follows.

Methacrylic acid: Available from Wako Pure Chemical Industries, Ltd.

Benzyl methacrylate: Available from Wako Pure Chemical Industries, Ltd.

Styrene: Available from Wako Pure Chemical Industries, Ltd.

Styrene macromer: “AS-6S” (available from Toagosei Co., Ltd.;concentration of effective ingredients: 50% by mass; number-averagemolecular weight: 6000)

Methoxy polyethylene glycol monomethacrylate (m=9): “NK ESTER M-90G”available from Shin-Nakamura Kagaku Kogyo Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=23): “NK ESTER TM-230G”available from Shin-Nakamura Kagaku Kogyo Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=30): “LIGHT ESTER 041MA”available from Kyoeisha Chemical Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=45): “NK ESTER M-450G”available from Shin-Nakamura Kagaku Kogyo Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=90): “NK ESTER M-900G”available from Shin-Nakamura Kagaku Kogyo Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=4): “NK ESTER M-40G”available from Shin-Nakamura Kagaku Kogyo Co., Ltd.

Methoxy polyethylene glycol monomethacrylate (m=120): Production ExampleI-13

Polypropylene glycol monomethacrylate: “PP-1000” available from NOFCorp. (molar number of addition of propylene glycol: 5)

Production Example I-13 Production of Methoxy Polyethylene GlycolMonomethacrylate (m=120)

Polyethylene glycol monomethyl ether (weight-average molecular weight:5312) having a molar number of addition of ethyleneoxide of 120 whichhad been melted at 80° C. was used to produce methoxy polyethyleneglycol monomethacrylate (m=120) as a product according to the methoddescribed in Example 1 of JP 11-228636A.

TABLE 1 Polymer solution 1 Polymer solution 2 Polymer solution 3Initially Initially Initially charged Dropping Dropping charged DroppingDropping charged Dropping Dropping monomer monomer monomer monomermonomer monomer monomer monomer monomer solution solution 1 solution 2solution solution 1 solution 2 solution solution 1 solution 2 (parts)(parts) (parts) (parts) (parts) (parts) (parts) (parts) (parts) Ionicmonomer (a-1) Methacrylic acid 72.8 18.2 72.8 18.2 72.8 18.2 Hydrophobicmonomer (a-2) Benzyl methacrylate 39.9 319.2 39.9 39.9 319.2 39.9 39.9319.2 39.9 Styrene Styrene macromer (AS-6S) 14.0 126.0 14.0 126.0 14.0126.0 Hydrophilic monomer (a-3) MPEGMA (m = 9) 14.0 112.0 14.0 MPEGMA (m= 23) 14.0 112.0 14.0 MPEGMA (m = 30) 14.0 112.0 14.0 MPEGMA (m = 45)MPEGMA (m = 90) Other monomers MPEGMA (m = 4) MPEGMA (m = 120) PPGMASolvent Methyl ethyl ketone 15.8 173.3 126.0 15.8 173.3 126.0 15.8 173.3126.0 Polymerization initiator V-65 5.6 1.4 5.6 1.4 5.6 1.4 Chaintransfer agent 2-Mercaptoethanol 0.4 2.4 0.7 0.4 2.4 0.7 0.4 2.4 0.7Polymer solution 4 Polymer solution 5 Polymer solution 6 InitiallyInitially Initially charged Dropping Dropping charged Dropping Droppingcharged Dropping Dropping monomer monomer monomer monomer monomermonomer monomer monomer monomer solution solution 1 solution 2 solutionsolution 1 solution 2 solution solution 1 solution 2 (parts) (parts)(parts) (parts) (parts) (parts) (parts) (parts) (parts) Ionic monomer(a-1) Methacrylic acid 72.8 18.2 72.8 18.2 72.8 18.2 Hydrophobic monomer(a-2) Benzyl methacrylate 39.9 319.2 39.9 39.9 319.2 39.9 29.4 235.229.4 Styrene Styrene macromer (AS-6S) 14.0 126.0 14.0 126.0 14.0 126.0Hydrophilic monomer (a-3) MPEGMA (m = 9) MPEGMA (m = 23) 24.5 196.6 24.5MPEGMA (m = 30) MPEGMA (m = 45) 14.0 112.0 14.0 MPEGMA (m = 90) 14.0112.0 14.0 Other monomers MPEGMA (m = 4) MPEGMA (m = 120) PPGMA SolventMethyl ethyl ketone 15.8 173.3 126.0 15.8 173.3 126.0 15.8 173.2 126.0Polymerization initiator V-65 5.6 1.4 5.6 1.4 5.6 1.4 Chain transferagent 2-Mercaptoethanol 0.4 2.4 0.7 0.4 2.4 0.7 0.4 2.4 0.7 Note MPEGMA:Methoxy polyethylene glycol monomethacrylate PPGMA: Polypropylene glycolmonomethacrylate (molar number of addition of propylene glycol: 5)

TABLE 2 Polymer solution 7 Polymer solution 8 Polymer solution 9Initially Initially Initially charged Dropping Dropping charged DroppingDropping charged Dropping Dropping monomer monomer monomer monomermonomer monomer monomer monomer monomer solution solution 1 solution 2solution solution 1 solution 2 solution solution 1 solution 2 (parts)(parts) (parts) (parts) (parts) (parts) (parts) (parts) (parts) Ionicmonomer (a-1) Methacrylic acid 72.8 18.2 72.8 18.2 72.8 18.2 Hydrophobicmonomer (a-2) Benzyl methacrylate 43.4 347.2 43.4 39.9 319.2 39.9 39.9319.2 39.9 Styrene Styrene macromer (AS-6S) 14.0 126.0 14.0 126.0 14.0126.0 Hydrophilic monomer (a-3) MPEGMA (m = 9) MPEGMA (m = 23) 10.5 84.010.5 MPEGMA (m = 30) MPEGMA (m = 45) MPEGMA (m = 90) Other monomersMPEGMA (m = 4) 14.0 112.0 14.0 MPEGMA (m = 120) 14.0 112.0 14.0 PPGMASolvent Methyl ethyl ketone 15.8 173.2 126.0 15.8 173.3 126.0 15.8 173.3126.0 Polymerization initiator V-65 5.6 1.4 5.6 1.4 5.6 1.4 Chaintransfer agent 2-Mercaptoethanol 0.4 2.4 0.7 0.4 2.4 0.7 0.4 2.4 0.7Polymer solution 10 Polymer solution 11 Polymer solution 12 InitiallyInitially Initially charged Dropping Dropping charged Dropping Droppingcharged Dropping Dropping monomer monomer monomer monomer monomermonomer monomer monomer monomer solution solution 1 solution 2 solutionsolution 1 solution 2 solution solution 1 solution 2 (parts) (parts)(parts) (parts) (parts) (parts) (parts) (parts) (parts) Ionic monomer(a-1) Methacrylic acid 72.8 18.2 72.8 18.2 78.4 19.6 Hydrophobic monomer(a-2) Benzyl methacrylate 18.9 151.2 18.9 46.9 375.2 46.9 Styrene 32.2257.6 32.2 Styrene macromer (AS-6S) 14.0 126.0 14.0 126.0 21.0 189.0Hydrophilic monomer (a-3) MPEGMA (m = 9) MPEGMA (m = 23) 35.0 280.0 35.07.0 56.0 7.0 MPEGMA (m = 30) MPEGMA (m = 45) MPEGMA (m = 90) Othermonomers MPEGMA (m = 4) MPEGMA (m = 120) PPGMA 17.5 140.0 17.5 SolventMethyl ethyl ketone 15.8 173.3 126.0 15.8 173.3 126.0 15.8 173.3 126.0Polymerization initiator V-65 5.6 1.4 5.6 1.4 5.6 1.4 Chain transferagent 2-Mercaptoethanol 0.4 2.4 0.7 0.4 2.4 0.7 0.4 2.4 0.7 Note MPEGMA:Methoxy polyethylene glycol monomethacrylate PPGMA: Polypropylene glycolmonomethacrylate (molar number of addition of propylene glycol: 5)

TABLE 3 Water-insoluble polymer (a) 1 2 3 4 5 6 7 8 9 10 11 12 Ionicmonomer (a-1) Methacrylic acid 13 13 13 13 13 13 13 13 13 13 13 14Hydrophobic monomer (a-2) Benzyl methacrylate 57 57 57 57 57 42 62 57 5727 67 Styrene 46 Styrene macromer (AS-6S) 10 10 10 10 10 10 10 10 10 1010 15 Hydrophilic monomer (a-3) MPEGMA (m = 9) 20 MPEGMA (m = 23) 20 3515 50 10 MPEGMA (m = 30) 20 MPEGMA (m = 45) 20 MPEGMA (m = 90) 20 Othermonomers MPEGMA (m = 4) 20 MPEGMA (m = 120) 20 PPGMA 25 Weight-average25112 28258 36348 28681 29800 28457 30720 33449 29465 26760 28385 27306molecular weight

Production Example II-1 Production of Pigment Water Dispersion 1

The water-insoluble polymer solution (solid content: 45.2%) obtained inProduction Example 1 and methyl ethyl ketone (MEK) were mixed with eachother in amounts of 44.2 parts and 36.1 parts, respectively, therebyobtaining a water-insoluble polymer MEK solution. The resultingwater-insoluble polymer MEK solution was charged into a 2 L-capacitydisper, and while stirring the solution at 1400 rpm, 176.8 parts ofion-exchanged water, 6.1 parts of a 5N sodium hydroxide aqueous solutionand 1.3 parts of a 25% ammonia aqueous solution were added thereto suchthat the degree of neutralization of the water-insoluble polymer bysodium hydroxide was adjusted to 85% and the degree of neutralization ofthe water-insoluble polymer by ammonia was adjusted to 40%. Theresulting reaction solution was stirred at 1400 rpm for 15 min whilecooling the solution in a water bath at 0° C. Then, 60 parts of carbonblack “MONARCH800” available from Cabot Corp., were added to thereaction solution, and the resulting mixture was stirred at 7000 rpm for3 h. The obtained pigment mixture was subjected to dispersion treatmentunder a pressure of 150 MPa by passing through a disperser“MICROFLUIDIZER M-110EH-30XP” (available from Microfluidics Corp.) 20times, thereby obtaining a dispersion treatment product. The resultingdispersion treatment product had a solid content of 25.0%.

A 2 L egg-plant shaped flask was charged with 324.5 parts of thedispersion treatment product obtained in the above step, and then 216.3parts of ion-exchanged water were added thereto (solid content: 15.0%),and the resulting mixture was held under a pressure of 0.09 MPa in awarm water bath adjusted at 32° C. for 3 h using a reduced-pressuredistillation apparatus “Rotary Evaporator N-1000S” available from TokyoRikakikai Co., Ltd., operated at a rotating speed of 50 rpm to removethe organic solvent therefrom. Further, the temperature of the warmwater bath was adjusted to 62° C., and the pressure was reduced to 0.07MPa, and the reaction solution was concentrated under this conditionuntil reaching a solid content of 25%.

The thus obtained concentrated solution was charged into a 500 mL anglerotor, and subjected to centrifugal separation using a high-speedcooling centrifuge “himac CR22G” (temperature set: 20° C.) availablefrom Hitachi Koki Co., Ltd., at 7000 rpm for 20 min. Thereafter, theresulting liquid layer portion was filtered through a 5 μm membranefilter “Minisart” available from Sartorius Inc.

Added to 300 parts of the resulting filtrate (pigment: 55.1 parts;water-insoluble polymer: 18.4 parts) were 48.4 parts of ion-exchangedwater, and further 18.4 parts of glycerin and 0.74 part of “Ploxel LVS”were added thereto, followed by stirring the resulting mixture at 70° C.or 1 h. The obtained mixture was cooled to 25° C. and then filteredthrough the above 5 μm filter, and further ion-exchanged water was addedto the resulting product to control a solid content thereof to 20.0%,thereby obtaining a pigment water dispersion 1.

The average particle size of the pigment-containing water-insolublepolymer particles contained in the resulting pigment water dispersion 1is shown in Table 4.

Production Examples II-2 to II-12 Production of Pigment WaterDispersions 2 to 12

The same procedure as in Production Example II-1 was repeated exceptthat the water-insoluble polymer solution 1 was replaced with therespective water-insoluble polymers 2 to 12, thereby obtaining pigmentwater dispersions 2 to 12. The results are shown in Table 4.

Production Example II-13 Production of Pigment Water Dispersion 13

The same procedure as in Production Example II-1 was repeated exceptthat the water-insoluble polymer solution 1 and the pigment werereplaced with the water-insoluble polymer 2 and a cyan pigment“CHROMOFINE BLUE 6338JC” available from Dainichiseika Color & ChemicalsMfg. Co., Ltd., respectively, and the rotating speed used in thecentrifugal separation step was changed from 7000 rpm to 3660 rpm,thereby obtaining a pigment water dispersion 13. The results are shownin Table 4.

Production Example II-14 Production of Pigment Water Dispersion 14

The same procedure as in Production Example II-1 was repeated exceptthat the water-insoluble polymer solution 1 and glycerin were replacedwith the water-insoluble polymer 2 and ion-exchanged water,respectively, thereby obtaining a pigment water dispersion 14. Theresults are shown in Table 4.

Production Example II-15 Production of Pigment Water Dispersion 15

The same procedure as in Production Example II-1 was repeated exceptthat the water-insoluble polymer solution 1 and the pigment werereplaced with the water-insoluble polymer 2 and a yellow pigment“FASTYELLOW 7414” (C.I. Pigment Yellow 74) available from Sanyo ColorWorks, Ltd., respectively, and the rotating speed used in thecentrifugal separation step was changed from 7000 rpm to 3660 rpm,thereby obtaining a pigment water dispersion 15. The results are shownin Table 4.

Production Example II-16 Production of Pigment Water Dispersion 16

The same procedure as in Production Example II-1 was repeated exceptthat the water-insoluble polymer solution 1 and the pigment werereplaced with the water-insoluble polymer 2 and a magenta pigment“CHROMOFINE RED 6111T” (C.I. Pigment Red 122) available fromDainichiseika Color & Chemicals Mfg. Co., Ltd., respectively, therebyobtaining a pigment water dispersion 16. The results are shown in Table4.

TABLE 4 Pigment/polymer Average particle Solid content Kind ofwater-insoluble polymer (a) ratio size (nm) (% by mass) Pigment waterdispersion 1 Water-insoluble polymer 1 75/25 87 20.0 Pigment waterdispersion 2 Water-insoluble polymer 2 75/25 85 20.0 Pigment waterdispersion 3 Water-insoluble polymer 3 75/25 91 20.0 Pigment waterdispersion 4 Water-insoluble polymer 4 75/25 85 20.0 Pigment waterdispersion 5 Water-insoluble polymer 5 75/25 83 20.0 Pigment waterdispersion 6 Water-insoluble polymer 6 75/25 84 20.0 Pigment waterdispersion 7 Water-insoluble polymer 7 75/25 86 20.0 Pigment waterdispersion 8 Water-insoluble polymer 8 75/25 82 20.0 Pigment waterdispersion 9 Water-insoluble polymer 9 75/25 82 20.0 Pigment waterdispersion 10 Water-insoluble polymer 10 75/25 103 20.0 Pigment waterdispersion 11 Water-insoluble polymer 11 75/25 94 20.0 Pigment waterdispersion 12 Water-insoluble polymer 12 75/25 81 20.0 Pigment waterdispersion 13 Water-insoluble polymer 2 75/25 93 20.0 Pigment waterdispersion 14 Water-insoluble polymer 2 75/25 85 20.0 Pigment waterdispersion 15 Water-insoluble polymer 2 75/25 124 20.0 Pigment waterdispersion 16 Water-insoluble polymer 2 75/25 112 20.0

Example 1 Production of Ink 1

The pigment water dispersion 1 was used to prepare a water-based ink 1for ink-jet printing. The water-based ink 1 was prepared from thefollowing composition A formulated such that the resulting ink containedthe pigment and the water-insoluble polymer particles B in amounts of 4%by mass and 5% by mass, respectively, and had a pH value of 9.0.

<Composition A>

Pigment water dispersion 1 26.67 parts Water-insoluble polymer particlesB: Anionic self- 11.36 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Glycerin (available from Wako Pure Chemical Industries, 8.67 parts Ltd.)Propylene glycol (available from Wako Pure Chemical 30 parts Industries,Ltd.) “SURFYNOL 104PG-50” (available from Nissin Chemical 0.60 partIndustry Co., Ltd., 2,4,7,9-tetramethyl-5-decyne-4,7- diol(50%),propylene glycol(50%)) “EMULGEN 120” (available from Kao Corp., 0.60part polyoxyethylene lauryl ether, HLB15.3) 1N Sodium hydroxide aqueoussolution 1.42 parts Ion-exchanged water 20.68 parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 1. Properties of the ink 1 are shown in Table5.

Meanwhile, the average boiling point of the organic solvent C as aweighted mean value of boiling points that were weighted by contents (%by mass) in the organic solvent C was 213° C. as the value calculatedfrom the following formula.

[glycerin content (% by mass)×boiling point of glycerin (290°C.)]+[propylene glycol content (% by mass)×boiling point of propyleneglycol (188° C.)]/[glycerin content (% by mass)+propylene glycol content(% by mass)]=[[0.1×290° C.]+[(0.3+0.003)×188° C.]]/[40.1+0.3+0.003]=213°C.

Meanwhile, in the above calculation formula, the numerical value “0.003”is a content (mass %) of propylene glycol derived from “SURFYNOL104PG-50”, and calculated from the following formula:

0.06 part×0.5=0.3 part=0.003% by mass in ink.

In addition, “0.1% by mass” as a glycerin content is the numerical valuecalculated from the following formulae:

Amount of pigment water dispersion 1 (glycerin content: 5% bymass)=26.67 parts;

Amount of glycerin derived from the pigment water dispersion1=26.67×0.05=1.33 parts;

Amount of glycerin compounded in ink=8.67 parts;

Total: 1.33 parts+8.67 parts=10.00 parts=0.1% by mass in ink.

Examples 2 to 7 and 10, and Comparative Examples 1 to 5 Production ofInks 2 to 7 and 10 to 15

The same procedure as in Example 1 was repeated except that the pigmentwater dispersion 1 was replaced with the respective pigment waterdispersions 2 to 13, thereby obtaining inks 2 to 7 and 10 to 15.However, the amounts of 1N sodium hydroxide aqueous solution andion-exchanged water compounded in the dispersion was adequately adjustedsuch that the total amount thereof was 22.1 parts, and the pH value ofthe ink after compounded was 9.0. Properties of the thus obtained inks 2to 7 and 10 to 15 are shown in Table 5.

Example 8 Production of Ink 8

The pigment water dispersion 6 was used to prepare an ink 8. The ink 8was prepared from the following composition B formulated such that theresulting ink contained the pigment and the water-insoluble polymerparticles B in amounts of 4% by mass and 5% by mass, respectively, andhad a pH value of 9.0.

<Composition B>

Pigment water dispersion 6 26.67 parts Water-insoluble polymer particlesB: Anionic self- 11.36 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Glycerin (available from Wako Pure Chemical Industries, 1.67 parts Ltd.)Diethylene glycol 37 parts “SURFYNOL 104PG-50” (available from NissinChemical 0.60 part Industry Co., Ltd.) “EMULGEN 120” (available from KaoCorp.) 0.60 part 1N Sodium hydroxide aqueous solution 1.36 partsIon-exchanged water 20.74 parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 8. Properties of the ink 8 are shown in Table5.

Meanwhile, the average boiling point of the organic solvent C ascalculated as a weighted mean value of boiling points that were weightedby contents (% by mass) in the organic solvent C was 247° C.

Example 9 Production of Ink 9

The pigment water dispersion 6 was used to prepare an ink 9. The ink 9was prepared from the following composition C formulated such that theresulting ink contained the pigment and the water-insoluble polymerparticles B in amounts of 4% by mass and 5% by mass, respectively, andhad a pH value of 9.0.

<Composition C>

Pigment water dispersion 6 26.67 parts Water-insoluble polymer particlesB: Anionic self- 11.36 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Glycerin (available from Wako Pure Chemical Industries, 11.67 partsLtd.) Propylene glycol (available from Wako Pure Chemical 24 partsIndustries, Ltd.) Dipropylene glycol monomethyl ether (available from 3parts Wako Pure Chemical Industries, Ltd.) “SURFYNOL 104PG-50”(available from Nissin Chemical 0.60 part Industry Co., Ltd.) “EMULGEN120” (available from Kao Corp.) 0.60 part 1N Sodium hydroxide aqueoussolution 1.41 parts Ion-exchanged water 20.69 parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 9. Properties of the ink 9 are shown in Table5.

Meanwhile, the average boiling point of the organic solvent C ascalculated as a weighted mean value of boiling points that were weightedby contents (% by mass) in the organic solvent C was 221° C.

Comparative Example 6 Production of Ink 16

The pigment water dispersion 6 was used to prepare an ink 16. The ink 16was prepared from the following composition D formulated such that theresulting ink contained the pigment and the water-insoluble polymerparticles B in amounts of 4% by mass and 5% by mass, respectively, andhad a pH value of 9.0.

<Composition D>

Pigment water dispersion 6 26.67 parts Water-insoluble polymer particlesB: Anionic self- 11.36 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Glycerin (available from Wako Pure Chemical Industries, 27.67 partsLtd.) Propylene glycol (available from Wako Pure Chemical 10 partsIndustries, Ltd.) “SURFYNOL 104PG-50” (available from Nissin Chemical0.60 part Industry Co., Ltd.) “EMULGEN 120” (available from Kao Corp.)0.60 part 1N Sodium hydroxide aqueous solution 1.36 parts Ion-exchangedwater 21.74 parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 16. Properties of the ink 16 are shown inTable 5.

Meanwhile, the average boiling point of the organic solvent ascalculated as a weighted mean value of boiling points that were weightedby contents (% by mass) in the organic solvent was 263° C.

Example 11 Production of Ink 17

The same procedure as in Example 2 using the pigment water dispersion 2was repeated except that the water-insoluble polymer particles B (as adispersion of the aqueous acrylic resin “Neocryl A1127”) used in the ink2 were replaced with 12.5 parts of a polyester resin emulsion “POLYESTERWR960” (available from Nippon Synthetic Chemical Industry Co., Ltd.; Tg:40° C.; solid content: 40% by weight), and the amount of ion-exchangedwater used therein was changed to 19.9 parts, thereby obtaining an ink17. Properties of the ink 17 are shown in Table 5.

Example 12 Production of Ink 18

The same procedure as in Example 2 using the pigment water dispersion 2was repeated except that the water-insoluble polymer particles B (as adispersion of the aqueous acrylic resin “Neocryl A1127”) used in the ink2 were replaced with 15.4 parts of a urethane resin emulsion “WBR-2018”(available from Taisei Fine Chemical Co., Ltd.; Tg: 20° C.; solidcontent: 32.5% by weight), and the amount of ion-exchanged water usedtherein was changed to 17.0 parts, thereby obtaining an ink 18.Properties of the ink 18 are shown in Table 5.

Example 13 Production of Ink 19

The same procedure as in Example 2 using the pigment water dispersion 2was repeated except that the water-insoluble polymer particles B (as adispersion of the aqueous acrylic resin “Neocryl A1127”) used in the ink2 were replaced with 12.5 parts of a vinyl chloride resin emulsion“VINYLBLAN 711” (available from Nissin Chemical Industry Co., Ltd.; Tg:20° C.; solid content: 40% by weight), and the amount of ion-exchangedwater used therein was changed to 19.0 parts, thereby obtaining an ink19. Properties of the ink 19 are shown in Table 5.

Example 14 Production of Ink 20

The pigment water dispersion 14 was used to prepare a water-based ink 20for ink-jet printing. The ink 20 was prepared from the followingcomposition E formulated such that the resulting ink contained thepigment and the water-insoluble polymer particles B in amounts of 4% bymass and 5% by mass, respectively, and had a pH value of 9.0.

<Composition E>

Pigment water dispersion 14 26.7 parts Water-insoluble polymer particlesB: Anionic self- 11.4 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Propylene glycol (available from Wako Pure Chemical 37 parts Industries,Ltd.) “SURFYNOL 104PG-50” (available from Nissin Chemical 0.6 partIndustry Co., Ltd.) “EMULGEN 120” (available from Kao Corp.) 0.6 part 1NSodium hydroxide aqueous solution 1.36 parts Ion-exchanged water 22.34parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 20. Properties of the ink 20 are shown inTable 5.

Meanwhile, the average boiling point of the organic solvent ascalculated as a weighted mean value of boiling points that were weightedby contents (% by mass) in the organic solvent was 192° C.

Example 15 Production of Ink 21

The pigment water dispersion 14 was used to prepare a water-based ink 21for ink-jet printing. The ink 20 was prepared from the followingcomposition F formulated such that the resulting ink contained thepigment and the water-insoluble polymer particles B in amounts of 4% bymass and 5% by mass, respectively, and had a pH value of 9.0.

<Composition F>

Pigment water dispersion 14 26.7 parts Water-insoluble polymer particlesB: Anionic self- 11.4 parts crosslinked aqueous acrylic resin “NeocrylA1127” (available from DSM NeoResins, Inc.; solid content: 44% by mass)Ethylene glycol (available from Wako Pure Chemical 38 parts Industries,Ltd.) “SURFYNOL 104PG-50” (available from Nissin Chemical 0.6 partIndustry Co., Ltd.) “EMULGEN 120” (available from Kao Corp.) 0.6 part 1NSodium hydroxide aqueous solution 1.36 parts Ion-exchanged water 21.34parts

The resulting mixed solution was filtered through the above 5 μm filter,thereby obtaining the ink 21. Properties of the ink 21 are shown inTable 5.

Meanwhile, the average boiling point of the organic solvent ascalculated as a weighted mean value of boiling points that were weightedby contents (% by mass) in the organic solvent was 200° C.

Examples 16 and 17 Production of Inks 22 and 23

The same procedure as in Example 1 was repeated except that the pigmentwater dispersion 1 was replaced with the respective pigment waterdispersions 15 and 16, thereby obtaining inks 22 and 23. However, theamounts of 1N sodium hydroxide aqueous solution and ion-exchanged watercompounded in the respective dispersions was adequately adjusted suchthat the total amount thereof was 22.1 parts, and the pH value of therespective inks after being compounded was 9.0. Properties of the thusobtained inks 22 and 23 are shown in Table 5.

TABLE 5 Weighed mean value of Kind of boiling point of organicwater-insoluble Pigment water solvent C Ink pH Viscosity polymer (a)dispersion *1 *2 Example 1 Ink 1 9.0 6.2 Water-insoluble Pigment water214° C. 213° C. polymer 1 dispersion 1 Example 2 Ink 2 9.0 6.6Water-insoluble Pigment water 214° C. 213° C. polymer 2 dispersion 2Example 3 Ink 3 9.0 6.6 Water-insoluble Pigment water 214° C. 213° C.polymer 3 dispersion 3 Example 4 Ink 4 9.0 6.4 Water-insoluble Pigmentwater 214° C. 213° C. polymer 4 dispersion 4 Example 5 Ink 5 9.0 6.6Water-insoluble Pigment water 214° C. 213° C. polymer 5 dispersion 5Example 6 Ink 6 9.0 6.8 Water-insoluble Pigment water 214° C. 213° C.polymer 6 dispersion 6 Example 7 Ink 7 9.0 6.6 Water-insoluble Pigmentwater 214° C. 213° C. polymer 7 dispersion 7 Example 8 Ink 8 9.0 5.7Water-insoluble Pigment water 248° C. 247° C. polymer 6 dispersion 6Example 9 Ink 9 9.0 6.9 Water-insoluble Pigment water 222° C. 221° C.polymer 6 dispersion 6 Example 10 Ink 10 9.0 5.9 Water-insoluble Pigmentwater 214° C. 213° C. polymer 2 dispersion 13 Example 11 Ink 17 9.0 6.4Water-insoluble Pigment water 214° C. 213° C. polymer 2 dispersion 2Example 12 Ink 18 9.0 6.5 Water-insoluble Pigment water 214° C. 213° C.polymer 2 dispersion 2 Example 13 Ink 19 9.0 6.6 Water-insoluble Pigmentwater 214° C. 213° C. polymer 2 dispersion 2 Example 14 Ink 20 9.0 6.9Water-insoluble Pigment water 193° C. 192° C. polymer 2 dispersion 14Example 15 Ink 21 9.0 6.8 Water-insoluble Pigment water 201° C. 200° C.polymer 2 dispersion 14 Example 16 Ink 22 9.0 6.2 Water-insolublePigment water 214° C. 213° C. polymer 2 dispersion 15 Example 17 Ink 239.0 6.8 Water-insoluble Pigment water 214° C. 213° C. polymer 2dispersion 16 Comparative Ink 11 9.0 6.2 Water-insoluble Pigment water214° C. 213° C. Example 1 polymer 8 dispersion 8 Comparative Ink 12 9.06.9 Water-insoluble Pigment water 214° C. 213° C. Example 2 polymer 9dispersion 9 Comparative Ink 13 9.0 7.3 Water-insoluble Pigment water214° C. 213° C. Example 3 polymer 10 dispersion 10 Comparative Ink 149.0 6.5 Water-insoluble Pigment water 214° C. 213° C. Example 4 polymer11 dispersion 11 Comparative Ink 15 9.0 6.5 Water-insoluble Pigmentwater 214° C. 213° C. Example 5 polymer 12 dispersion 12 Comparative Ink16 9.0 6.4 Water-insoluble Pigment water 264° C. 263° C. Example 6polymer 6 dispersion 6 Note *1: Calculated value except for propyleneglycol derived from “SURFYNOL 104PG-50”. *2: Calculated value inclusiveof propylene glycol derived from “SURFYNOL 104PG-50”.

Experimental Example 1 Dot Size (Preparation of Printed Matters)

The commercially available ink-jet printer “GX-2500” (available fromRicoh Company, Ltd.; piezoelectric-type) was loaded with the respectivewater-based inks obtained in Examples 1 to 17 and Comparative Examples 1and 4 to 6, and the Duty 10% printing was carried out on an ordinarygloss coated paper “OK Topcoat+” (available from Oji Paper Co., Ltd.;basis weight: 104.7 g/m²) at 23° C. and 50% RH under the printingcondition of “gloss coated paper; Kirei; no color matching”, therebyobtaining printed matters 1 to 4. The dot size was measured using ahandy-type image evaluation system “Model No. PIAS-II” available fromQEA Inc., to read out the diameter (μm) of the dots on the Duty 10%printed portions on the obtained print matters. The results are shown inTable 6.

As the dot size increases, the number of overlapped dots increases sothat the optical density and gloss become higher.

Experimental Example 2 Spread of Dot Size

As the dot size when printed on a low-water absorbing recording mediumincreases, the ink is more excellent in picking upon one-pass printing.The dot size has a correlation with the viscosity of the ink whenconcentrated.

The dot size was evaluated by the viscosity of the ink whenconcentrated. As the viscosity of the ink is reduced, the ink is moreexcellent in spread of the dot size as well as ejection property.

(Preparation of Concentrated Ink)

Using a reduced pressure dryer, the inks obtained in Examples 1 to 10and Comparative Examples 1 and 4 to 6 were respectively concentrated at60° C. under a pressure of 210 mmHg, thereby preparing inks having adegree of concentration of 60%. Meanwhile, the degree of concentrationof the respective inks was calculated from the following formula.

Degree of concentration=(mass of ink after concentrated/mass of initialink)×100

(Viscosity of Respective Concentrated Inks)

Using a rheometer “MCR301” (available from Anton Paar GmbH; using a coneplate “CP50-1”), the above concentrated inks were subjected tomeasurement of a stress at 32° C. at an increased shear rate of 100,200, 300, 400, 500, 600, 700, 800, 900 and 1000. The thus obtainedstress-shear rate plot curve was fit by a least squares method to passthrough an origin, and a gradient of a straight line portion of the thusfit curve was defined as the viscosity of the concentrated ink. Theresults are shown in Table 6.

Experimental Example 3 Optical Density (Preparation of Printed Matters)

The commercially available ink-jet printer “GX-2500” (available fromRicoh Company, Ltd.; piezoelectric-type) was loaded with the respectivewater-based inks obtained in Examples 1 to 10 and Comparative Examples 1and 4 to 6, and the A4 solid image (monochrome) printing was carried outon an ordinary gloss coated paper “OK Topcoat+” (available from OjiPaper Co., Ltd.; basis weight: 104.7 g/m²) and a polyester film“LUMIRROR T60” (available from Toray Industries Inc.; thickness: 125 μm)at 23° C. and 50% RH under the printing condition of “gloss coatedpaper; Kirei; no color matching”, thereby obtaining the followingprinted matters 1 to 4.

The printed matter 1 obtained after being printed on the ordinary glosscoated paper was allowed to stand at 23° C. and 50% RH for 24 h and thendried.

The printed matter 2 obtained after being printed on the ordinary glosscoated paper was allowed to stand at 23° C. and 50% RH for 30 min andthen dried.

The printed matter 3 obtained after being printed on the ordinary glosscoated paper was heated for 15 s on an analogue hot plate “(NINOS) NA-2”(available from As One Corp.) heated to 100° C. and then dried.

The printed matter 4 obtained after being printed on the polyester filmwas heated for 5 min on the same hot plate as used above and then dried.

Meanwhile, the printed matters 2 and 3 thus dried were immediatelysubjected to evaluation of a rub fastness thereof.

(Measurement of Optical Density)

The optical density values of the respective solid image portions on theprinted matters 1 and 4 were measured using an optical densitometer“SPECTROEYE” (available from Gretag Macbeth Corp.) in a measuring modeof (DIN, Abs). The results are shown in Table 6. As the measured valueincreases, the ink is more excellent in optical density.

Experimental Example 4 Gloss

The gloss (20°) of the solid image portion on the printed matter 1 wasmeasured by a gloss meter “HANDY GLOSSMETER, Model No.: PG-1M” availablefrom Nippon Denshoku Industries Co., Ltd. The results are shown in Table6. As the measured value increases, the ink is more excellent in gloss.

Experimental Example 5 Rub Fastness

The printed matters 1 to 3 were subjected to a rub fastness test inwhich each printed matter was rubbed with a cotton “BEMCOT M-3”(available from Asahi Kasei Corp.) as a friction material using aSutherland-type Ink Rub Tester “AB-201” (available from Tester SangyoCo., Ltd.) under a load of 2000 g 100 times (reciprocating operations).The rubbed printed matter was observed by the naked eyes and evaluatedaccording to the following ratings. The results are shown in Table 6.

(Evaluation Ratings)

◯: No peeling occurred

x: Peeling occurred

TABLE 6 Spread of Optical density 20° Gloss Rub fastness Dot size dotsize Printed matters Printed Printed matters (μm) (mPa · s) 1 4 matter 11 2 3 Example 1 Ink 1 62 135 1.90 2.06 17.1 ◯ ◯ ◯ Example 2 Ink 2 77  822.30 — 23.0 ◯ ◯ ◯ Example 3 Ink 3 63 118 2.29 — 29.0 ◯ ◯ ◯ Example 4 Ink4 72 110 2.33 2.08 32.5 ◯ ◯ ◯ Example 5 Ink 5 70 120 2.23 2.04 27.6 ◯ ◯◯ Example 6 Ink 6 73  73 2.31 — 24.2 ◯ ◯ ◯ Example 7 Ink 7 63 130 2.01 —20.3 ◯ ◯ ◯ Example 8 Ink 8 71 112 2.37 — 29.2 ◯ ◯ ◯ Example 9 Ink 9 66124 2.30 — 24.3 ◯ ◯ ◯ Example 10 Ink 10 84  58 2.36 — 20.8 ◯ ◯ ◯ Example11 Ink 17 84  75 2.13 — 24.5 ◯ ◯ ◯ Example 12 Ink 18 81  92 2.21 — 22.3◯ ◯ ◯ Example 13 Ink 19 71 105 2.24 — 21.5 ◯ ◯ ◯ Example 14 Ink 20 70114 2.14 — 20.2 ◯ ◯ ◯ Example 15 Ink 21 82 103 2.20 — 21.5 ◯ ◯ ◯ Example16 Ink 22 83  70 1.80 — 24.0 ◯ ◯ ◯ Example 17 Ink 23 70 120 1.71 — 22.3◯ ◯ ◯ Comparative Ink 11 55 140 1.73 — 14.1 ◯ ◯ ◯ Example 1 ComparativeInk 12 Normal ink ejecting operation was not possible and thereforeevaluation of printed matters was Example 2 not possible. ComparativeInk 13 Normal ink ejecting operation was not possible and thereforeevaluation of printed matters was Example 3 not possible. ComparativeInk 14 53 170 1.81 1.93 16.3 ◯ ◯ ◯ Example 4 Comparative Ink 15 48   242¹⁾ 2.08 — 17.4 ◯ ◯ ◯ Example 5 Comparative Ink 16 64 122 2.31 — 22.4 ◯ ◯X Example 6 Note ¹⁾ Viscosity at a degree of concentration of 65%; notconcentrated to 60%.

As apparently shown in Table 6, it was confirmed that the inks obtainedin Examples 1 to 17 were excellent in total evaluation of the dot sizeon a low-water absorbing recording medium, spread of the dot size,optical density, gloss and rub fastness as compared to the inks obtainedin Comparative Examples 1 and 4 to 6.

Reference Example

The same ink-jet printer as used in Experimental Example 2 was loadedwith the respective water-based inks 1 to 9, 17 to 23, 11 and 14 to 16obtained in Examples 1 to 9 and 11 to 17 and Comparative Examples 1 and4 to 6, respectively, and the A4 solid image (monochrome) printing wascarried out on plain paper “4200” (available from Fuji Xerox Co., Ltd.)at 23° C. and 50% RH under the printing condition of “plain paper; Fast;no color matching”, thereby obtaining the following printed matters 5and 6.

The printed matter 5 thus obtained after printing was allowed to standat 23° C. and 50% RH for 24 h and then dried, whereas the printed matter6 thus obtained after printing was allowed to stand at 23° C. and 50% RHfor 30 s and then dried.

The printed matter 5 was subjected to the same measurement andevaluation for optical density as in Experimental Example 2. Also, theprinted matters 5 and 6 were subjected to the same rub fastness test asin Experimental Example 4. The results are shown in Table 7.

TABLE 7 Reference Optical density Rub fastness Example Printed matter 5Printed matter 5 Printed matter 6 Ink 1 0.92 ◯ ◯ Ink 2 0.90 ◯ ◯ Ink 30.90 ◯ ◯ Ink 4 0.90 ◯ ◯ Ink 5 0.89 ◯ ◯ Ink 6 0.90 ◯ ◯ Ink 7 0.92 ◯ ◯ Ink8 0.92 ◯ ◯ Ink 9 0.91 ◯ ◯ Ink 17 0.90 ◯ ◯ Ink 18 0.90 ◯ ◯ Ink 19 0.90 ◯◯ Ink 20 0.89 ◯ ◯ Ink 21 0.91 ◯ ◯ Ink 22 0.94 ◯ ◯ Ink 23 0.69 ◯ ◯ Ink 110.97 ◯ ◯ Ink 14 0.94 ◯ ◯ Ink 15 0.94 ◯ ◯ Ink 16 0.93 ◯ ◯

The water-based inks for ink-jet printing according to the presentinvention were excellent in spread of the dot size on a low-waterabsorbing recording medium, optical density, gloss and rub fastness(refer to Table 6). However, as is apparent from Table 7, when printedon a recording medium having a large water absorption, i.e., ahigh-water absorbing recording medium, the inks 1 to 9 obtained inExamples 1 to 9 and the inks 17 to 23 obtained in Examples 11 to 17 wereexcellent in rub fastness to a similar extent, but poor in opticaldensity, as compared to the inks 11 and 14 to 16 obtained in ComparativeExamples 1 and 4 to 6.

Thus, it was recognized that the ink-jet printing method according tothe present invention can exhibit excellent effects, in particular, whenapplied to a low-water absorbing recording medium.

INDUSTRIAL APPLICABILITY

In the ink-jet printing method according to the present invention, whenprinted on a low-water absorbing recording medium, the printed images orcharacters are excellent in spread of the dot size, optical density,gloss and rub fastness. Therefore, the present invention can be suitablyused as an ink-jet printing method in extensive applications.

1. An ink-jet printing method for printing images or characters on arecording medium using a water-based ink for ink-jet printing, in whichthe water-based ink comprises pigment-containing water-insoluble polymerparticles A, water-insoluble polymer particles B, an organic solvent Cand water; the water-insoluble polymer particles A comprises awater-insoluble polymer (a) containing a constitutional unit derivedfrom an ionic monomer (a-1), a constitutional unit derived from anaromatic ring-containing hydrophobic monomer (a-2) and a constitutionalunit derived from a hydrophilic monomer (a-3) represented by the formula(1):

wherein R¹ is a hydrogen atom or a methyl group; R² is a hydrogen atom,an alkyl group having not less than 1 and not more than 20 carbon atomsor a phenyl group whose hydrogen atom may be substituted with an alkylgroup having not less than 1 and not more than 9 carbon atoms; and mrepresents an average molar number of addition of ethyleneoxy groups andis a number of not less than 7 and not more than 100, the constitutionalunit derived from the hydrophilic monomer (a-3) being present in anamount of not less than 13% by mass and not more than 45% by mass on thebasis of whole constitutional units in the water-insoluble polymer (a);the organic solvent C comprises one or more organic solvents having aboiling point of 90° C. or higher, and has an average boiling point of250° C. or lower as a weighted mean value of boiling points that areweighted by a content (% by mass) of respective organic solvents in theorganic solvent C; and the recording medium has a water absorption ofnot less than 0 g/m² and not more than 10 g/m² as measured in a purewater contact time of 100 ms.
 2. The ink-jet printing method accordingto claim 1, wherein the organic solvent C comprises at least onecompound selected from the group consisting of a polyhydric alcohol anda polyhydric alcohol alkyl ether.
 3. The ink-jet printing methodaccording to claim 1, wherein a content of the at least one compoundselected from the group consisting of a polyhydric alcohol and apolyhydric alcohol alkyl ether in the organic solvent C is 80% by massor more.
 4. The ink-jet printing method according to claim 1, whereinthe organic solvent C comprises at least one compound selected from thegroup consisting of propylene glycol, diethylene glycol and dipropyleneglycol monomethyl ether, and glycerin.
 5. The ink-jet printing methodaccording to claim 1, wherein the aromatic ring-containing hydrophobicmonomer (a-2) comprises at least one monomer selected from the groupconsisting of a styrene-based monomer, an aromatic group-containing(meth)acrylate and a styrene-based macromonomer.
 6. The ink-jet printingmethod according to claim 1, wherein the ionic monomer (a-1) is acarboxylic acid monomer.
 7. The ink-jet printing method according toclaim 1, wherein a content of the constitutional unit derived from theionic monomer (a-1) in the water-insoluble polymer (a) is not less than3% by mass and not more than 40% by mass.
 8. The ink-jet printing methodaccording to claim 1, wherein a content of the constitutional unitderived from the aromatic ring-containing hydrophobic monomer (a-2) inthe water-insoluble polymer (a) is not less than 20% by mass and notmore than 80% by mass.
 9. The ink-jet printing method according to claim1, wherein a mass ratio of the component (a-1) to a sum of the component(a-2) and the component (a-3) {component (a-1)/[component(a-2)+component (a-3)]} is not less than 0.03 and not more than 0.50.10. The ink-jet printing method according to claim 1, wherein a contentof the pigment in the water-based ink is not less than 1% by mass andnot more than 15% by mass.
 11. The ink-jet printing method according toclaim 1, wherein a content of the water-insoluble polymer (a) in thewater-based ink is not less than 0.5% by mass and not more than 6% bymass.
 12. The ink-jet printing method according to claim 1, wherein acontent of the water-insoluble polymer particles B in the water-basedink is not less than 1.0% by mass and not more than 10% by mass.
 13. Theink-jet printing method according to claim 1, wherein a content of theorganic solvent C in the water-based ink is not less than 20% by massand not more than 60% by mass.
 14. The ink-jet printing method accordingto claim 1, wherein the pigment-containing water-insoluble polymerparticles A in the water-based ink has an average particle size of notless than 40 nm and not more than 150 nm.
 15. The ink-jet printingmethod according to claim 1, wherein the water-insoluble polymerparticles B in the water-based ink has an average particle size of notless than 10 nm and not more than 300 nm. 16.-21. (canceled)